Add OID to inquire GSS cred impersonator name

Add OID to inquire GSS cred impersonator name

In the krb5 GSS mechanism, add support in gss_inquire_cred_by_oid()
for inquiring the impersonator name of a credential object, using OID
1.2.840.113554.1.2.2.5.14.

[ghudson@mit.edu: edited code slightly; added documentation; expanded
commit message]

ticket: 8548 (new)