Refactor: various: Limit xmlSetProp() to pcmk__xe_set_attr_force()

This facilitates refactoring private data allocation/deallocation.

This is basically a backport of e8bb4fad. However, the 2.1 branch still
has some extra legacy functions that are confusing -- especially
related to v1 patchsets. Rather than try to be certain that ACLs and
change tracking are disabled so that we can use crm_xml_add(), we create
the new function pcmk__xe_set_attr_force() that simply wraps
xmlSetProp().

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