Ensure that sentinels are called during 'accept-process-output'.

Ensure that sentinels are called during 'accept-process-output'.

When we're trying to notify a process about a status change, we need
to ignore the SIGCHLD pipe temporarily, otherwise the code would
likely not run into the timeout case that's necessary for a status
change to happen.

* src/process.c (wait_reading_process_output): Ignore the SIGCHLD pipe
when notifying a process about a status change.
* test/src/process-tests.el (process-tests/sentinel-called)
(process-tests/sentinel-with-multiple-processes): New unit tests.