Refactor: libcrmcommon: New pcmk__xe_set()
To replace crm_xml_add().
Also add an include of results.h to ipc.h. For some strange reason, this
commit causes test-headers.sh to break due to missing symbol crm_exit_t
in that header. Similar with some other include changes.
Signed-off-by: Reid Wahl <nrwahl@protonmail.com>