session: Rename perform_kerberos() -> perform_gssapi()

Suggested by Simo for clarity

Closes #1240
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
session: Use gss_localname() to resolve unix user names

This keeps us using the GSSAPI API which is cleaner that jumping
over to krb5 API once resolving the user.

Suggested by Simo Sorce

Closes #1241
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>