• Handle SIGPIPE in gssd

We've got some cases where the other end of gssd's AF_LOCAL socket gets
closed, resulting in an error (and SIGPIPE) when it tries to do I/O to it.
Closing without cleaning up means the next time nfsd starts up, it hangs,
unkillably; this allows gssd to handle that particular error.

• Limit the retry cound in gssd_syscall to 5.

The default is INT_MAX, which effectively means forever. And it's an
uninterruptable RPC call, so it will never stop.

The two changes mitigate the problem.

Reviewed by: macklem
MFC after: 2 weeks
Sponsored by: iXsystems Inc.
Differential Revision: https://reviews.freebsd.org/D19153