Changes in process.c

[Eli Zaretskii]

People who make changes in process.c related to keyboard input and
wait_reading_process_output, please remember that there are two
branches in process.c, one for systems that support async
subprocesses, the other for those which don't (only MS-DOS in the
latter class).  Making changes only in one of them is going to break
something.

Btw, unless there are good reasons to have the new hold-keyboard
functions in process.c, I would suggest to move them to keyboard.c,
since this will eliminate the need to have 2 implementations of them.