Implement RFC 3645 Section 3.1.1 ret_flags check in GSS-API client

Implement RFC 3645 Section 3.1.1 ret_flags check in GSS-API client

After gss_init_sec_context() completes, verify that both MUTUAL and
INTEG flags are set in ret_flags.  RFC 3645 Section 3.1.1 requires
the client to abandon the algorithm if either flag is missing, as
the security context would not provide mutual authentication or
message integrity.

Also fix uninitialized gss_name_t variable in dst_gssapi_initctx()
that could cause undefined behavior if gss_import_name() fails and
the cleanup path calls gss_release_name() on the uninitialized
value.