wxFFile/wxLog on OSX

Hi Andreas,

The first problem I've found whilst playing with out Mac port is in the
logging code. Before fixing it I just wanted to run it past you in case
you have any thoughts...

When pgAdmin starts with default settings, it almost certainly won't
have permissions to write to the default logfile. That in itself should
be easy to fix - I'll figure out the appropriate way to get the user's
home directory on Mac and stick it there.

The real problem is that wxFFile appears to be logging it's failure to
open the log file through the logging interface - if I try it in a
simple test app it pops up a 'cannot open' message box, however in
pgAdmin this gets sent back to the logger, thus causing a loop and
eventual overflow.

The simple solution would be to re-implement the WriteLog using
fopen/fprintf/fclose I guess, but do you see a better way to solve this?

Regards, Dave.