Use up-to-date time in wait_reading_process_output

In “Avoid duplicate calls to current_timespec” (2015-07-05)
we started caching current_timespec results in NOW.
However, this was buggy: we updated NOW only when the timeout was
nonzero, but the timeout can be set temporarily to zero in several
places in wait_reading_process_output (such as when checking for
process status changes), which would cause us to never update NOW
and therefore never detect that a timeout happened.

Also, this caching was wrong even in principle: since we call
Lisp code from wait_reading_process_output, substantial amounts
of time can pass, and we can be left using an outdated NOW and
incorrectly not time out.  Also, nowadays we can use
monotonic_coarse_timespec which is fast, and which is better
anyway because it’s immune to manual clock changes.

Co-authored-by: Spencer Baugh <sbaugh@janestreet.com>
* src/process.c (wait_reading_process_output):
Stop caching the current realtime.
Instead, use the coarse monotonic clock without caching.

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