Pre-assign fixed loglevels

Pre-assign fixed loglevels

Some early LTTng-UST adopters brought to my attention that the way
tracepoint loglevels are currently specified in LTTng-UST might be too
relax for its own good. If each application define their own loglevel
names/values, it will become difficult to use the loglevels to select
"trace verbosity" in a system-wide manner.

Now that I come to think of it, it might make sense to pre-define a set
of supported loglevels, similarly to syslog(3). However, given that
tracing sometimes targets debug levels that are more fine-grained than
in the case of logs, I would propose to split the "debug" loglevel into
sub-categories.

My current thought is to simply just allow these loglevels. I doubt that
letting application developers specify extra loglevels on top of this
would be that useful, and it would certainly be more confusing.

In the list below, lower numbers means "low verbosity", higher numbers
means "high verbosity, debug-style information".

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>