Limit use of IAKERB

Limit use of IAKERB

Add the GSS_C_MA_NOT_DFLT_MECH attribute to IAKERB, and filter out
mechs with that attribute from the SPNEGO and gss_acquire_cred()
default mechanisms.

Add a -iakerb option to gss-server and pass it when performing IAKERB
tests.  Also add tests using the wrong password, to verify that
gss_acquire_cred_with_password() fails with the wrong password when
using SPNEGO.

ticket: 8021