Problem: principals are looked up unconditionally
with the GSS_C_NT_HOSTBASED_SERVICE name type.

Solution: Add two new socket options to set the name type
for ZMQ_GSSAPI_PRINCIPAL and ZMQ_GSSAPI_SERVICE_PRINCIPAL:

ZMQ_GSSAPI_PRINCIPAL_NAMETYPE
ZMQ_GSSAPI_SERVICE_PRINCIPAL_NAMETYPE

They take an integer argument which must be one of
ZMQ_GSSAPI_NT_HOSTBASED (0) - default
ZMQ_GSSAPI_NT_USER_NAME (1)
ZMQ_GSSAPI_NT_KRB5_PRINCIPAL (2)

These correspond to GSSAPI name types of:
GSS_C_NT_HOSTBASED_SERVICE
GSS_C_NT_USER_NAME
GSS_KRB5_NT_PRINCIPAL_NAME

Fixes #2542