Issue 173: add child process log messages to main log file

Slave processes were not using the master process log file.
On Unix this is relying on the server.logfile variable being available
to the slave processes through fork(), and reopening the logfile
in the slaves (on every log event).
On Windows we don't use server.logfile and require an explicity call
to setLogFile.
I resorted to explicitly passing the logfile to the slaves as a
command line argument, so the logfile argument (and logging) can be
available to the slave before qfork and globals setup have completed.

Writing to the same file atomically from multiple processes requires
using CreateFile with FILE_APPEND_DATA, instead of fopen, which provides
atomicity on Unix but not on Windows.

Also changed the implementation to not reopen the logfile on every
log event, and not flushing the file on every write. Performance is
dramaticaly improved this way.