BUG/MINOR: startup: close pidfd and free global.pidfile in handle_pidfile()

BUG/MINOR: startup: close pidfd and free global.pidfile in handle_pidfile()

After master-worker mode refactoring, global.pidfile is only used in
handle_pidfile(), which opens the provided file and writes the PID into it. So,
it's more appropriate to perform the close(pidfd) and ha_free(&global.pidfile)
also in this function.

This commit prepares the fix of the pidfile creation, as it's created now very
early, when we are not sure, that process has successfully started. In
master-worker mode handle_pidfile() can be called in the master process context.
So, let's make it accessible from other compilation units via global.h.

This should be backported only in 3.1.