Make mountd generate warning log messages when administrative controls exports entire file systems

[rmacklem]

Summary

When "administrative controls" (which are exports of subdirectories
within a NFS server's local file system) are used, they export the
entire local server file system. (The subdirectory only applies to
the Mount protocol used for NFSv3 mounts.)

To minimize the risk that this causes confusion w.r.t. what is exported
to NFS client(s), this patch generates warning messages for these.
Only one message is generated for each server local file system.
The messages can be silenced via a new "-A" command line option.

The mountd.8 man page will be patched via a separate commit.

Test Plan

Tested with an exports file that has multiple
administrative controls in it.

[emaste]

usr.sbin/mountd/mountd.c

276

we could start sneaking C99 in, if you're so inclined
static bool warn_admin = true

[rmacklem]

usr.sbin/mountd/mountd.c

276

Sun RPC has its own bool definition that
is not compatible with stdbool.h.
(There is probably some way to make them
co-exist, but I think I'll resist and just stick
with the style it is written in.)

[markj]

This looks reasonable to me, thank you.