util: Add base64 from Heimdal's roken

util: Add base64 from Heimdal's roken

Add the base64 code from libroken into libutil, using the src/external
framework.

Create a new, non-installed directory, to hold util headers, rather than
continuing to stuff everything into afs/

Change-Id: I6b743a3702c3bb07ac798392475e7e067765f238
Reviewed-on: http://gerrit.openafs.org/2900
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>