core: Make notification debugging a (lot) easier

Looking up the notification type from a list in the sources is
boring. Especially when Nagios should just tell you what type
it's trying to print.

Since it's kind of useful for other code as well (such as for
example the some-day-to-be-invented NERD notification channel),
we make the id-to-name function publicly accessible.

With this patch we also change the NOTIFICATION_CUSTOM macro
from 99 to 8, for implementation reasons.

This means people from the future who look at debug logs from
today may get confused. The opposite isn't true ofcourse, since
the debug logs in the future will hold a string explaining what
kind of notification we're trying to send without requiring the
manual decoding that's done today.

Signed-off-by: Andreas Ericsson <ae@op5.se>