kpwd: add support for multiple GIDs

Storage-authzdb supports the use of multiple GIDs.

While the Kpwd file allows the login-line to have
a comma-separated list of GIDs, these are not used
anywhere in the gPlazma plugin for Kpwd, but are
only accessed in the Kpwd Login strategy class.

Also, the passwd line still takes only one GID.

This patch adds support throughout KPWD for multiple
gids.  This is particularly useful, for instance,
in allowing authorization of the webadmin pages
without having to create a cell-specific gPlazma
module (all one needs to do is append the webadmin
admin gid to the user's gids, and include kpwd mapping
as sufficient/optional in the gPlazma config).

There are two things I am unsure of.

1.  If UserMetaDataProviderFnal is still used,
    should it report all the gids, or just the primary?
    (I have left it for now in the latter state).
2.  Is the main() entry point on the KAuthFile class
    still used?  If so, I have kept the
    modify method such that it will only allow
    a single GID for the moment (not understanding
    how this works or where it is used).

Testing: Tested writes and webadmin login, varying the presence
or absence of the admin-gid on my login and passwd lines.

Target: master
Patch: https://rb.dcache.org/r/7159
Require-book: yes
Require-notes: yes
Acked-by: Gerd Behrmann

BOOK
RELEASE NOTES

Multiple Gids are now supported on both the
login and passwd lines of the KPWD file, and
are accessed by the map plugin for Kpwd.
These should be given as a comma-separated
list, with the primary gid first.