mount.nfs: Shorter timeout for TCP connects

mount.nfs: Shorter timeout for TCP connects

The standard TCP connect timeout on Linux is 75 seconds, which can be
too long in some cases.  The timeout itself can be altered on a system-wide
basis, but we'd like mount to have it's own connect timeout that's tunable,
and defaults to a shorter value.

The get_socket() function is a utility function that does TCP connects for
getport, clnt_ping, and other functions.  Add logic there to use a
non-blocking connect() and select() in order to time out a connect
operation that's taking too long.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Neil Brown <neilb@suse.de>