mount_nfs.c: Add an NFS mount option to set a port# for Mount
Normally mount_nfs acquires the port# for the NFS server's
Mount protocol (mountd(8)) via rpcbind/portmapper for
NFSv3 mounts.
This patch adds a new mount option, so that the mount
command can specify the port# and avoid using rpcbind
for NFSv3 mounts.
The new option is called "mountport" since that is what Linux
calls their mount option for the same semantics.
PR: 282481
Reviewed by: delphij
Tested by: Ronald Minnich <rminnich@gmail.com>
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D47743