basic/log: CODE_FUNCTION → CODE_FUNC

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.