mountd.c: Define a new -a command line option

Bugzilla PR#282995 reported that, when a file system was
exported with the "-alldirs" flag, the export succeeded even
if the directory path was not a server file system mount point.

This behaviour for "-alldirs" was only documented in the
Example section of exports(5) and had not been enforced
since FreeBSD2. (A patch applied between FreeBSD1 and
FreeBSD2 broke the check for file system mount point.)

Since the behaviour of allowing the export has existed since
FreeBSD2, the concensus on a mailing list was that it would
be a POLA violation to change it now.
Therefore, this patch adds a new "-a" mountd command line
option to enforce a check for the exported directory being a
server file system mount point.

PR: 282995
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D48137