MFC: r347583
Replace global list for grouplist with list(s) for each exportlist element.

In mountd.c, the grouplist structures are linked into a single global
linked list headed by "grphead". The only use of this linked list is
to free all list elements when the exportlist elements are also all being
free'd at the time the exports are being reloaded.
This patch replaces this one global linked list head with a list head in
each exportlist structure, where the grouplist elements for that exported
file system are linked.
The only change is that now the grouplist elements are free'd with the
associated exportlist element as they are free'd instead of all grouplist
elements being free'd after the exportlist elements are free'd. This
change should have no effect in practice.
This is being done, since a future patch that will add a "-I" option for
incrementally updating the exports in the kernel needs to know which
grouplist elements are associated with each exported file system and
having them linked into a list headed by the exportlist element does that.

PR: 237860