Fix wait_reading_process_output wait_proc hang

Fix wait_reading_process_output wait_proc hang

* src/process.c (read_process_output): Track bytes read from
a process.
(wait_reading_process_output): If called recursively through
timers and/or process filters via accept-process-output, it is
possible that the output of wait_proc has already been read by
one of those recursive calls, leaving the original call hanging
forever if no further output arrives through that fd and no
timeout has been set.  Fix that by using the process read
accounting to keep track of how many bytes have been read and
use that as a condition to break out of the infinite loop and
return to the caller as well as to calculate the proper return
value (if a wait_proc is given that is).

* src/process.h (struct Lisp_Process): Add nbytes_read to track
bytes read from a process.