GSSEAP_TRACE prints returns from major entry points

GSSEAP_TRACE prints returns from major entry points

Especially with SPNEGO, it's often the case that the caller does not
always make the GSSEAP errors available.  So, when GSSEAP_TRACE is
set, print the results of init_sec_context, acquire_cred and
accept_sec_context.

Introduce gssEapTrace for additional tracing/debugging.