Release buffer after export_name

The low-level `export_name` function was missing a call to
`gss_release_buffer`, which could have resulted in a memory leak.

This is now fixed.
Normalize 'creds' argument name

Previously, a couple methods used 'cred' instead of 'creds'
as argument names.  Now, all methods use 'creds' (including
'acceptor_cred' becoming 'acceptor_creds')