Both read and write operations support variants where either a) a single
call is made to the undelying stream implementation and returned buffer
length may be less than requested, or b) calls are repeated until requested
amount of data is collected, shorter amount is returned only in case of
EOF or error.

These operations are available from the level of C support functions to be
used by other C modules to implementations of Python methods to be used in
user-facing objects.

The rationale of these changes is to allow to write concise and robust
code to work with *blocking* streams of types prone to short reads, like
serial interfaces and sockets. Particular object types may select "exact"
vs "once" types of methods depending on their needs. E.g., for sockets,
revc() and send() methods continue to be "once", while read() and write()
thus converted to "exactly" versions.

These changes don't affect non-blocking handling, e.g. trying "exact"
method on the non-blocking socket will return as much data as available
without blocking. No data available is continued to be signaled as None
return value to read() and write().

From the point of view of CPython compatibility, this model is a cross
between its io.RawIOBase and io.BufferedIOBase abstract classes. For
blocking streams, it works as io.BufferedIOBase model (guaranteeing
lack of short reads/writes), while for non-blocking - as io.RawIOBase,
returning None in case of lack of data (instead of raising expensive
exception, as required by io.BufferedIOBase). Such a cross-behavior
should be optimal for MicroPython needs.

This allows to enable the options which aren't enabled in the normal unix
config (as unix port is no longer an enable-all port).

modwebrepl truncates password to 9 chars, and that led people to confusion.

To use frozen bytecode make a subdirectory under the unix/ directory
(eg frozen/), put .py files there, then run:

    make FROZEN_MPY_DIR=frozen

Be sure to build from scratch.  The .py files will then be available for
importing.

When an mpy file is frozen it must know the values of certain
configuration variables.  This patch provides an explicit check in the
generated C file that the configuration variables are what they are
supposed to be.

That was the intent for the initial user setup, but didn't work before
due to lwIP issues. Enable now that they're fixed.

That one was missing in the module, even if it was available in the
vfs object. The change consist of adding the name and preparing the
call to the underlying vfs module, similar to what was already
implemented e.g. for remove.

Rename is useful by itself, or for instance for a safe file replace,
consisting of the sequence:

    write to a temp file
    delete the original file
    rename the temp file to the original file's name

Calling it from lwIP accept callback will lead incorrect functioning
and/or packet leaks if Python callback has any networking calls, due
to lwIP non-reentrancy. So, instead schedule "poll" callback to do
that, which will be called by lwIP when it does not perform networking
activities. "Poll" callback is called infrequently though (docs say
every 0.5s by default), so for better performance, lwIP needs to be
patched to call poll callback soon after accept callback, but when
current packet is already processed.

It allows to "import foo.bar", but not "from foo import bar".

Since the ``stop`` parameter has been dropped.

For example, the following code now works with a file on the SD card:

    f = open('test', 'rb') # test must be 1024 bytes or more in size
    f.seek(511)
    f.read(513)

Also works for writing.

Fixes issue #1863.

It now supports \n, \r and \r\n as newline separators.

Adds 56 bytes to stmhal and 80 bytes to unix x86-64.

Fixes issue #1689.

If custom C code uses the I2C busses then it needs access to these
structures for i2c_init().

This allows PYBv3 to use PWM for LED(1) and LED(2).

Address printed was truncated anyway and in general confusing to outsider.
A line which dumps it is still left in the source, commented, for peculiar
cases when it may be needed (e.g. when running under debugger).

'=' is pretty natural character for tail, and gives less dense picture
where it's easier to see what object types are actually there.

OSError's are now printed like:

    OSError: [Errno 1] EPERM

but only if the string corresponding to the errno is found.

Actual loading of .mpy files isn't tested.

In some compliation enviroments (e.g. mbed online compiler) with
strict standards compliance, <math.h> does not define constants such
as M_PI.  Provide fallback definitions of M_E and M_PI where needed.

If an OSError is raised with an integer argument, and that integer
corresponds to an errno, then the string for the errno is used as the
argument to the exception, instead of the integer.  Only works if
the uerrno module is enabled.

These are typical consumers of large chunks of memory, so it's useful to
see at least their number (how much memory isn't clearly shown, as the data
for these objects is allocated elsewhere).

Fixes issue #2065.

The timeval struct can be defined by system C headers.