Refactor: libcrmcommon: functionize message creation

create_request_adv() and create_reply_adv() duplicated a lot of code.
Functionize the shared code as pcmknew_message_as(), with a (currently
unused) pcmknew_message() macro wrapper.

The new function takes a server type as an argument, and sets the message type
(PCMK__XA_T) to that server's primary message type. As of this commit, callers
pass pcmk_ipc_controld to preserve previous behavior.