[krbdev.mit.edu #7593] Implement support for xdr_u_quad_t in gssrpc

[Simo Sorce via RT]

GLibc introduced support for xdr_u_quad_t which encodes a 64 bit
unsigned integer.
This is quite handy given 64 bit unsigned quantities are quite common
these days on 64bit machines.

Attached find a possible implementation, I built it for the gss-proxy
project that transitioned from using glibc's rpc library to gssrpc.