export: Ensure that we free struct exportent->e_uuid

Currently, the exportent->e_uuid is initialised in
support/nfs/exports.c:parseopts(), but it is never freed.

Also ensure that exportent->e_uuid is duplicated correctly in
dupexportent().

Adjusted to account for the new export_free() helper.

Also, e_uuid points to memory that is always allocated with strdup(3),
not with xstrdup().  Thus it must be freed via free(3) and not via
xfree().

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>