os: manage/dissolve Signal_rpc_member in class

Instead of, passing responsibility to manage and dissolve Signal_rpc_member
objects at a corresponding entrypoint to the user, hand over entrypoint's
reference to the constructor, and do it in the constructor resp. destructor
of the class.

Fixes #1022
block: fix race in generic block driver component

Fixes #1023
Vancouver: Clarify license headers

All source codes of the glue code between Vancouver and Genode that were
based on existing GPL code had already stated that Genode Labs is not
the sole owner of the code. To make the distinction between the Genode
OS framework and the glue code more clear, this patch removes the text
"is part of the Genode OS framework" from the glue code.