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/mech/gss_cred.c

@@ -223,3 +223,4 @@ gss_import_cred(OM_uint32 * minor_status,
     return major;
 
 }
+