mountd: Use a dynamic buffer for storing lists of gid's

mountd: Use a dynamic buffer for storing lists of gid's

Previously, in auth_unix_gid, group lists were stored in an array of
hard-coded length 100, and in the situation that the group lists for a
particular call were too large, the array was swapped with a dynamically
allocated/freed buffer.  For environments where users are commonly in
a large number of groups, this isn't an ideal approach.

Instead, use malloc/realloc to grow the list on an as-needed basis.

Signed-off-by: Sean Finney <sean.finney@sonyericsson.com>
Signed-off-by: Steve Dickson <steved@redhat.com>