gssapi: add NAMETYPE socket options

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

src/options.hpp

@@ -199,6 +199,10 @@ namespace zmq
         std::string gss_principal;
         std::string gss_service_principal;
 
+	//  Name types GSSAPI principals
+	int gss_principal_nt;
+	int gss_service_principal_nt;
+
         //  If true, gss encryption will be disabled
         bool gss_plaintext;