API: libcrmcommon: New xml_names.h header to replace msg_xml.h

msg_xml.h now includes msg_xml_compat.h if PCMK_ALLOW_DEPRECATED is
undefined or is defined as 1. Otherwise it includes only xml.h.

msg_xml_compat.h now explicitly includes xml_names.h.

To facilitate review, the includes from other files won't be updated
until the next commit. So this commit probably won't compile by itself.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>