Bugfix: Problems exposed by unit tests for f_signal.

Bugfix: Problems exposed by unit tests for f_signal.

Fix ordering of functions.
Compare using "== -1" rather than "< 0" because the POSIX standard says "-1" rather than "less than zero".

The f_signal_read() function has several problems:
- Should be passing a pointer rather than an array to poll().
- Needs to check data_poll.revents.
- Missing the parameter checking.