util.c: Made write_pidfile write the correct PID on MinGW/Msys

This workaround fixes an issue on MinGW/Msys regarding the Perl
testsuite scripts not being able to signal or control the server
processes. The MinGW Perl runtime only sees the Msys processes and
their corresponding PIDs, but sockfilt (and other servers) wrote the
Windows PID into their PID-files. Since this PID is useless to the
testsuite, the write_pidfile function was changed to search for the
Msys PID and write that into the PID-file.