BUG/MINOR: mworker: fix validity check for the pipe FDs

BUG/MINOR: mworker: fix validity check for the pipe FDs

Check if master-worker pipe getenv succeeded, also allow pipe fd 0 as
valid. On FreeBSD in quiet mode the stdin/stdout/stderr are closed
which lets the mworker_pipe to use fd 0 and fd 1. Additionally exit()
upon failure to create or get the master-worker pipe.

This needs to be backported to 1.8.