Symlinkconfig

This pull request contains code that can help users manage their config. 
Specifically it allows one to manage switches by having the PATH (and other 
environment variables) always pointing to the same directory, and symlinks 
the correct one.


The encouraged workflow is as follows:
````shell
opam switch myfavouritecompilerflavour
eval `opam config symlink`
````

The main difference is that `opam config profile` modifies the files system 
(and thus the whole system) while `opam config env` modifies the environment 
(and thus the current process only. Also, symlinks survive reboots.


This current prototype is not complete (it doesn't handle the system switch) 
and probably work environment specific (i.e. not tested on different 
shells/bases/setups). Also, all paths are hard-coded.

The pull request is to gather feedback and discussions.