log_event() create log file if it does not exist (0001509)

log_event() create log file if it does not exist

This is a regression from issue 0019642. When MantisBT tries to write an
event to a log file that does not already exist, a warning is printed
and the file is not created even if PHP process has write access to the
directory. The event is added to the PHP system log.

Instead of relying on is_writable(), the code now calls error_log() with
errors suppressed, then checks the result of the function call to
determine if the operation was successful or not.

Fixes 0025734