Implement afsconf_GetRXGKKey

Implement afsconf_GetRXGKKey

Also afsconf_GetLatestRXGKKey, as a side effect, since we want to have
a single getkey function both for getting encrypting and decrypting keys;
a kvno/enctype pair of 0/0 indicates that the "get latest" behavior is
desired.

Implement both functions in terms of an internal helper that takes
as an argument the type of key to look for in the KeyFileExt.  We
can reuse these helpers wholesale for per-fileserver keys, later.

This also requires implementing an ordering on the quality of the
different RFC 3961 enctypes (which are stored as the subtype of keys
of type afsconf_rxgk). This is subject to debate on the actual
ordering, but since the IANA enctype registry changes rarely, just
assign a full ordering on the standardized (symmetric!) enctypes.
Implement this via a new function, rxgk_enctype_better, in
rxgk_crypto_rfc3961.c.

Introduce a new header file, rxgk_types.h, so we can avoid including
the entire rxgk.h header in cellconfig.p.h.

Change-Id: I81389b21238fd6588cc4381b026816005f81a30c
Reviewed-on: https://gerrit.openafs.org/11099
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>