Allow default options in /etc/exports

  Implement default options in /etc/exports, to fix a long-standing wishlist
  bug in Debian. (The user claims the syntax matches that of OpenBSD.) This
  makes it possible to write "/srv/www -sync,no_subtree_check host1 host2 host3"
  instead of having to write (sync,no_subtree_check) over and over and over
  again, driving the administrator slowly mad. Such option lines can be
  placed anywhere on the line, and affects anything after them (I do not
  know if OpenBSD allows this). The patch is slightly convoluted in order to
  avoid triggering spurious warnings; for instance, we want
  "/srv/www -sync host1" to trigger a warning, but not "/srv/www
  -sync,no_subtree_check host1" or "/srv/www -sync host1(no_subtree_check)".

  There was also a suggestion for a truly global (ie. per-file) option list,
  but this seemed like the safest bet, given that it matches that of other
  implementations.

  Also, the man page is updated with information on the new possibilities,
  and an example.

Signed-off-by: Steinar H. Gunderson <sesse@debian.org>
Signed-off-by: Neil Brown <neilb@suse.de>