tweaked fcron logging to file

tweaked fcron logging to file
- only enabled for fcron via a command line option (rather than fcron.conf)
  I doubt people need this feature for command line tools, and we would need to manage the log file perms more closely to allow all tools to write to it
- reworked log.c/log.h: more sensible function names, better error checks
- changed the line format when logging to files/console
- added documentation on logging to file