w.i.p. remote reg and pub/sub

[uwiger]

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First sketch of remote registration and gproc_ps subscriptions.

The idea is that registering remote pids shouldn't be a problem in itself, but all regs must go through the gproc server, can only have local scope, and the API function should be distinctly only for non-local pids.

TODO: add ensure_... and unreg functions etc. as well as test cases.