Try to give a more descriptive error message for the pilot error of

Try to give a more descriptive error message for the pilot error of
attempting to export the non-root of a filesystem with -alldirs.  This
pilot error seems to be very common, and the "could not remount" error
message doesn't give much hints about the real reason.  See the old PR
below for an example.

While i was at it, make it possible to entirely omit the often
annoying error message in that case by specifying the "quiet" exports
flag.  This allows to specify something like

/cdrom	-alldirs,ro,quiet	<where to export to>

which will silently fail if nothing is mounted under /cdrom, but do
the rigth thing as soon as you mount something.

While doing this, i've put the embedded example in the exports(5) man
page into a subsection of its own as it ought to be.

Thanks for Paul Southworth for reminding me about this problem.

PR:		bin/4448
MFC after:	1 month