Add a warning message w.r.t. using soft, intr options for NFSv4 mounts

[rmacklem]

Summary

NFSv4 mounts are broken by either the "soft" or "intr" options.
This is noted in the BUGS section of "man mount_nfs".
However, a couple of recent PRs that were broken this way
indicates that generating a warning message for one of
these mounts is appropriate.

Test Plan

Tried NFSv4 mounts with/without "soft" and "intr", to see that
the warning is generated, as required.

[emaste]

Please reference the PRs in the commit message. The message seems reasonable to me for now, although should we consider just disallowing soft/intr entirely for NFSv4?