Refactor: libcrmcommon: Best practices in log_data_element()

• Add doxygen block.
• Use defined constants for diff prefixes. The previous assignments to prefix_m[1] made sense only if the passed-in prefix was "+ " or "- "; they don't make sense with an arbitrary prefix.
• Replace diff prefixes with NULL in some callers. The options flags can fully determine the diff prefix within log_data_element(), in those cases.
• Remove redundancy in check for "no children" or "diff marker".
• Short-circuit recursive call if neither xml_log_option_diff_plus nor xml_log_option_diff_minus is set. The options flag never changes in this case, so nothing will be logged.

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