Use fstat(2) to check if command file is a FIFO

Use fstat(2) rather than stat(2) to check whether the command file is a
named pipe.  This fixes a race condition in the case where the command
file is recreated after the NSCA-ng server has open(2)ed it.
Apply another test suite fix for GNU Hurd

On GNU Hurd, opening the FIFO sometimes fails with EINTR (even though no
signal has been delivered).