fix umount so that it correctly handles the Unmount RPC for TCP or NFSv4 mounts

[rmacklem]

Summary

This patch modifies umount(8) so that, when NFS mounts are being unmounted, it uses TCP for the
Unmount RPC if the NFSv3 mount is over TCP and does not do any Unmount RPC for NFSv4.
Without this patch, umount(8) always does an Unmount RPC over UDP.

Test Plan

Do umounts for NFSv3,UDP NFSv3,TCP and NFSv4 mounts and look at the packet trace
via wireshark to see that the correct Unmount RPC is done or not done at all.

[cperciva]

Subject to the s/char */const char */ on line 329, this fixes the long-wait-on-umount I saw on nfsv4. I don't have any non-nfsv4 mounts, so I can't test those.

sbin/umount/umount.c

329 ↗

(On Diff #22162)

This needs to be 'const char *' since we're assigning constant strings to it.

[rmacklem]

Add the "const" recommended by cperciva.