logger: fix callback log level and output

[geyslan]

commit e99ef72

logger: pass raw libbpf print output

Due to libbpf output being escaped, we should pass the raw output to the
consumer, letting them decide how to format it, since them might want
to, for example, use it in a logger or print directly to stderr.

Reported-by: @yanivagman

commit 9f21a9d

logger: fix callback log level

Fixes: #311
Fixes: #313

[yanivagman]

LGTM

[geyslan]

@yanivagman I guess we can't output it as literal strings (to be used by a json structured logger): uber-go/zap#963 (comment)

---

In the lib we must pass the original output via callbacks. The consumer is who decides how to unescape, trim... and print it.

If you agree, I'll remove the commit d0119cf, leaving the original one that fixes the level issue, so we can discuss how to tackle the output on tracee.

--- EDIT

After discussion, we decided to provide a raw output string for callbacks.