gssapi: credential store extensions (#451)

Implement the GSS-API credential store API extensions defined by MIT here:

https://k5wiki.kerberos.org/wiki/Projects/Credential_Store_extensions

Note: we kill off gss_acquire_cred_ext() here. This was never a public API,
although mechanisms could have implemented it and I briefly used it in my
BrowserID prototype mechanism. gss_acquire_cred_ext_from() occupies the place
in the dispatch table where gss_acquire_cred_ext() used to, but this structure
was never visible outside Heimdal (i.e. it is only used by internal
mechanisms);

(Mechanisms that need to accept arbitrary key/value dictionaries from
applications should now implement gss_acquire_cred_from().)

lib/gssapi/libgssapi-exports.def

@@ -9,13 +9,13 @@ EXPORTS
 	__gss_krb5_nt_principal_name_oid_desc	DATA
 	__gss_c_attr_stream_sizes_oid_desc	DATA
         __gss_c_attr_local_login_user           DATA
-        __gss_c_cred_certificate_oid_desc       DATA
-        __gss_c_cred_password_oid_desc          DATA
 	gss_accept_sec_context
 	gss_acquire_cred
+	gss_acquire_cred_from
 	gss_acquire_cred_with_password
 	gss_add_buffer_set_member
 	gss_add_cred
+	gss_add_cred_from
 	gss_add_cred_with_password
 	gss_add_oid_set_member
         gss_authorize_localname
@@ -91,6 +91,7 @@ EXPORTS
 	gss_set_sec_context_option
 	gss_sign
 	gss_store_cred
+	gss_store_cred_into
 	gss_test_oid_set_member
 	gss_unseal
 	gss_unwrap