Refactor: libcrmcommon: Best practices in log_xml_node_and_children()

...and pcmk__xml_log().

Add doxygen blocks, limit variable scope, improve variable name, use
const where appropriate, improve spacing and indentation, use
pcmk__str_update(), etc.

Rename to log_xml_node(), since we log children only if the
xml_log_option_children flag is set.

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