Fix mountd to handle getgrouplist() not returning groups[0] == groups[1].

Prior to r174547, getgrouplist(3) always returned a groups list with
element 0 and 1 set to the basegid argument, so long as ngroups was > 1.
Post-r174547 this is not the case. r328304 disabled the deduplication that
removed the duplicate, but the duplicate still does not occur unless the
group for a user in the password database is also entered in the group
database.
This patch fixes mountd so that it handles the case where a user specified
with the -maproot or -mapall exports option has a getgrouplist(3) groups
list where groups[0] != groups[1].
Found while testing another mountd patch.

MFC after: 2 weeks