Better handling of desired_mechs

If no explicit allowed mechanism is set in configuration just ask
GSSAPI for a list of known mechanisms and use that. Do not try to
artificially acquire credentials as ultimatily all that does is
just call gss_inidicate_mechs() internally.

Do not store the result of gss_inidicate_mechs() on cfg->allowed_mechs
as that would lead to a leak given that cfg->allowed_mechs is allocated
on a memory pool, while gss_inidate_mechs()s results are not.

Closes #44

Signed-off-by: Simo Sorce <simo@redhat.com>