Leaving on process group misfunctioning

[Sébastien Saint-Sevin]

When we leave a process that belongs to multiple groups, it is unlinked. This should be the case only when that process belongs to one group and only one. Otherwise the gen_server is no more able to track EXIT messages to properly clean the other groups that process belongs to when it will die.

[Roberto Ostinelli]

Thank you, merged in.