Properly handle timeout values over ULLONG_MAX

--timeout, -t option accepts very large values, over ULLONG_MAX. When
this happens the inotify scripts will default the timeout value to
ULLONG_MAX and wait for that amount of time (which seems highly unlikely
that someone would want that); as of now, the current behaviour is to
display an error message and exit when errno != 0 after the strtoul
call.