Problem: there is no test coverage for GSSAPI.

Solution: add a test structured like the CURVE test.

The test is not built if libzmq is not configured with
--with-libgssapi_krb5. It will report SKIPPED status
if the required environment is missing (see below).

Environment:  KRB5_KTNAME and KRB5_CLIENT_KTNAME
environment variables must point to a keytab file
containing creds for a host-based test principal
(see comment at top of source for details).
Kerberos must be configured and a KDC containing the
test principal must be running, otherwise the test
will fail/hang.

N.B. For now, the test must use the same principal for
both client and server roles because it seems impossible
to set them to different principals when they are
threads in the same process.  Once one principal is
cached in credential cache, attempts to acquire creds
for a different "desired name" seem to be ignored and
the cached principal is used instead.