Add auto_fast_armor realm variable

Add auto_fast_armor realm variable

When auto_fast_armor is true and no FAST ccache name is provided in
the GIC options, use anonymous PKINIT to acquire an armor ticket into
a temporary memory ccache.  Refactor krb5int_fast_as_armor() and add
an armor_ccache parameter so that we can pass in the memory ccache
without modifying the caller's GIC options.

[ghudson@mit.edu: edited commit messages, documentation, and comments;
refactored some code changes for simplicity]

ticket: 9226 (new)