Problem: if client sets the ZMQ_GSSAPI_PRINCIPAL to a valid
principal, authentication fails.

When an application sets ZMQ_GSSAPI_PRINCIPAL, whether as a
client or a server, libzmq internally calls gss_acquire_cred()
with cred_usage=GSS_C_ACCEPT.  This cred_usage setting is for
acceptors (servers) only, thus it doesn't work for initiators
(clients).

Solution: Change the cred_usage parameter to GSS_C_BOTH to allow
initiators to set ZMQ_GSSAPI_PRINCIPAL.