Clean up log file handling. Fixes bug #63

Clean up log file handling.  Fixes bug #63

We now open the log file from the option parsing (-l file)
OR in mainconfig.c.  That way, the code in log.c can assume that
there is ALWAYS a log file, and it doesn't need to open one.  This
simplifies log.c substantially.

We also moved the syslog "openlog" code from log.c to mainconfig.c
This again makes it simpler.

On HUP, the hup_mainconfig() function takes care of re-opening the
log file.  This is so that the log.c functions don't have to do it.