BibSched: fix guess_apache_process_user_from_ps

• guess_apache_process_user_from_ps was mistakenly returning process names instead of user names. This is now fixed.

• Improves the corresponding regular expression to ignore strings such as "<defunct>" that might appear on "ps" output.

Co-authored-by: Ludmila Marian <ludmila.marian@gmail.com>