Fix impersonate_name to work with interposers

Fix impersonate_name to work with interposers

This follows the same modifications applied to
gss_acquire_cred_with_password() when interposer plugins were
introduced.

[ghudson@mit.edu: minor whitespace changes; initialize out_mcred in
spnego_gss_acquire_cred_impersonate_name() since it is released in the
cleanup handler]

ticket: 8280 (new)