text-based mount.nfs: Create helpers for invoking mount(2) system call

Add simple helper functions that invoke the mount(2) system call for
text-based mounts.  These look the same right now, but the NFSv2/v3 helper
will become more complex over the following patches as we implement version
and transport protocol fallback.

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