Split conf_extlibs.py and conf_platform.py.

conf_extlibs.py was over 1100 lines. Move check_readline, check_curses,
and _check_curses (plus the WITH_READLINE arg) into conf_terminal.py,
grouping terminal/console I/O library detection together. Split
conf_probes.py from conf_platform.py.

Assisted-by: Claude

Author: nascheme