Use rxkad_crypt for inter-volser traffic, if asked

Use rxkad_crypt for inter-volser traffic, if asked

Add a -s2scrypt option to the volume server, with possible options:

  * never -- the existing behavior

  * always -- switch to using afsconf_ClientAuthSecure, which uses
    rxkad_crypt, for ForwardVolume calls.

  * inherit -- encrypt inter-server traffic if the causal client
    connection is encrypted.  This has the effect of "inheriting" the
    "-encrypt" flag given to "vos release", for example.

Thanks to Jeffrey Altman for pointers and to Andrew Deason for noting
the existence of rxkad_GetServerInfo.

[mmeffie@sinenomine.net fix assertion and style update.]

Change-Id: Ia295ba3f29a8494c8250a480fb26594468d2116a
Reviewed-on: https://gerrit.openafs.org/11349
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Thomas Keiser <tkeiser@gmail.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>