Since commit a427213f, the special glib value "help" does not work
anymore as a helper (listing the list of available flags).
This means that to use GIMP_DEBUG environment variable, one has to
either know them all by heart or check the app/gimp-log.c file.
This commit still leaves "help" as a normal flag for GIMP_LOG_HELP
domain in GIMP, but creates instead "list-all" as the new helper value.

Moreover as a fallback, setting a random value to GIMP_DEBUG with no
valid flags in it will also output the list of available flags. This
way, one doesn't even have to remember a specific string to obtain the
list.