Refactor: libcrmcommon: New pcmk__xml_write_temp_file()

To replace save_xml_to_file(). Note that the new function
unconditionally prefixes the file name with a temp dir, while the old
function did so only if the filename was NULL. This is the reason why
the call to pcmkget_tmpdir() is dropped from pcmkdigest_xml().

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