add git-remote-p2p-annex

add git-remote-p2p-annex

Added git-remote-p2p-annex, which allows git pull and push to P2P networks
provided by external commands.

This is a refactor of git-remote-tor-annex, and should just work. Except
possibly for quirks with the address parsing. I've checked that the address
parsing basically works.

One thing I don't understand is why git-remote-tor-annex removes "/*" from
the end of the address. The git history does not provide any hints. So I
didn't make git-remote-p2p-annex do the same. Maybe that is needed in some
situation? But, a P2P address could contain "/", so removing it would be a
problem. I can't see anything in gitremote-helpers(7) about why the url
might get such a thing added to the end of it. My guess is that is not
needed for tor either (but does no harm there since onion addresses never
contain "/").

At this point, the implementation of generic P2P transports needs only
remotedaemon support.