- if multiple gssapi mechanisms in ssh_gssapi_mechanisms, separate them

- if multiple gssapi mechanisms in ssh_gssapi_mechanisms, separate them
  by ',' in the list
- added some debug messages in ssh_gssapi_mechanisms to show which mechs
  are chosen and why
- initialize gss_buffer_desc variables to NULL
- avoid calling gss_delete_sec_context() and gss_release_name() with
  mechglue because there are problems with name binding between the
  different gssapi libraries that cause memory management problems