basic/log: CODE_FUNCTION → CODE_FUNC

systemd.journal-fields(7) documents CODE_FUNC=. Internally, we were
inconsistent: sd_journal_print uses CODE_FUNC=, log.h has CODE_FUNCTION=,
python-systemd and bootchart also used CODE_FUNC=, when they were internal.
Most external projects use sd_journal_* functions, so CODE_FUNC=,
python-systemd still uses CODE_FUNC=, as does systemd-bootchart, and
independent reimplementations in golang-github-coreos-go-systemd, qtbase,
network manager, glib, pulseaudio. Hence, I don't think there's much
choice.

src/basic/log.c

@@ -500,7 +500,7 @@ static int log_do_header(
                  line ? "CODE_LINE=" : "",
                  line ? 1 : 0, line, /* %.0d means no output too, special case for 0 */
                  line ? "\n" : "",
-                 isempty(func) ? "" : "CODE_FUNCTION=",
+                 isempty(func) ? "" : "CODE_FUNC=",
                  isempty(func) ? "" : func,
                  isempty(func) ? "" : "\n",
                  error ? "ERRNO=" : "",