gssapi: Allow logging in as users listed in "k5principals" extra field. This also enables other passdb extra fields for gssapi mechanism.
gssapi: Allow logging in as users listed in "k5principals" extra field.
This also enables other passdb extra fields for gssapi mechanism.

Based on patch by Sam Morris.

dscho/dovecot,dscho/dovecot,dscho/dovecot,dscho/dovecot,dscho/dovecot