jtag/tcl.c cleanup -- split out "adapter.c"

Clean up the jtag/tcl.c file, which was one of the biggest and
messiest ones in that directory.  Do it by splitting out all the
generic adapter commands to a separate "adapter.c" file (leaving
the "tcl.c" file holding only JTAG utilities).

Also rename the little-used "jtag interface" to "adapter_name", which
should have been at least re-categorized earlier (it's not jtag-only).

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>