add P2PAnnex constructor

add P2PAnnex constructor

This is for p2p-annex:: urls that will use the new generic P2P
transport.

In addressCredsFile, threw in an url encoding of any non-alphanumeric
characters that are in the address. This is to avoid any possible path
traversal attacks via a p2p-annex:: url, since the address part of it
could contain any characters. And, went ahead and did the same url
encoding of tor-annex:: urls, even though tor onion addresses are all
alphanumerics, on the off chance that might avoid a similar problem.
(It does not seem likely enough to treat it as a security hole.)