bos: Do encryption if requested

Commit d008089a79 didn't replace the processing of the aencrypt
flag passed to the GetConn() function, causing all bos connections
to be un-encrypted.  This causes "addkey" to fail with an error
from the server, and "listkeys" to silently ignore the -showkey
option to display keys.

Set the AFSCONF_SECOPTS_ALWAYSENCRYPT flag, and don't set
AFSCONF_SECOPTS_FALLBACK_NULL since fallback is not acceptable if
the caller requested enrcyption.

Change-Id: I6a15fa20c172ab2ddfc6494fbcd62ec6bc7fc442
Reviewed-on: http://gerrit.openafs.org/10043
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>