unable to control stack depth shown by logger

[tharvik]

Currently, log.lvl uses a skip argument to select how many stackframes to drop. log.lvl{d,f} both set it at 3 before forwarding the call. Sadly, there is no way to set the relevant call depth from the outside. That's useful in order to provide preciser logging: sometimes, you want all the logger's call in a given function to prepend some identification string, which can be easily done by having some logger oneliner in the preamble; depending on how much a project using onet grows, it usually wants to build a logger upon the existing logging infrastracture, so, control over skip would be nice.

One trivial (but wrong) fix would be to have a var Skip uint in the log pkg, but as it is shared by everyone, some other parts might except a certain skip value, not the one you set.

Using a logger object, providing an interface similar to the one existing would allow for this use case separation.