Fix S4U2Self against non-FAST KDCs

When we added FAST TGS support in 1.11, we broke S4U2Self against KDCs
which don't support FAST, because the S4U2Self padata is only present
within the FAST request.  For now, duplicate that padata in the outer
request so that both FAST and non-FAST KDCs can see it.

ticket: 7791
target_version: 1.11.5
tags: pullup
Avoid malloc(0) in SPNEGO get_input_token

If we read a zero-length token in spnego_mech.c's get_input_token(),
set the value pointer to NULL instead of calling malloc(0).

ticket: 7794 (new)
Better keysalt docs

Add a new section to kdc_conf.rst to describe keysalt lists, and
update other documentation to better distinguish enctype lists from
keysalt lists.

ticket: 7608
target_version: 1.12
tags: pullup