pyexec: Add event-driven variant pyexec_friendly_repl().

pyexec_friendly_repl_process_char() and friends, useful for ports which
integrate into existing cooperative multitasking system.

Unlike readline() refactor before, this was implemented in less formal,
trial&error process, minor functionality regressions are still known
(like soft&hard reset support). So, original loop-based pyexec_friendly_repl()
is left intact, specific implementation selectable by config setting.