Fix log level "everything" and make it equal to "debug"

As Holger points out "logging level XXX everything" wasn't working, as
it sets category->loglevel to 0, which is checked in osmo_vlogp() and
will never get logged.
Added a ring buffer log target to store the last N log messages.

The log target can be used via log alarms and show alarms.
Why? This feature was proposed/requested at
http://openbsc.osmocom.org/trac/wiki/Tasks/ErrorLogTarget
All messages use the same amount of space, prioritizing simplicity.
Renames as per tnt's request.
UI changed to reflect number of usable messages, as per zecke's request.