Reduce 'td' argument to 'cred' (struct ucred) argument in those functions:

• in_pcbbind(),
• in_pcbbind_setup(),
• in_pcbconnect(),
• in_pcbconnect_setup(),
• in6_pcbbind(),
• in6_pcbconnect(),
• in6_pcbsetport().

"It should simplify/clarify things a great deal." --rwatson

Requested by: rwatson
Reviewed by: rwatson, ume