Disassociate public key tags from peer names.

Disassociate public key tags from peer names.

Peer names are being used for too much now.  Originally, they /only/
identified the public key.  The peer management services muddy the issue
greatly by mapping peer names to database records.

I propose to resolve this mess in favour of the peer management
services.  The ADD command now takes an optional argument naming the
public key to use to authenticate the peer (defaulting to the peer name,
as before).  The `connect' service consults the peer database to find
which key to pass to the server.

This change modifies the server to support a new `-key' option to ADD,
and use the value of this option in preference to the peer name if it's
supplied.  It also updates the `connect' service to use this option if
necessary.  Finally, there's documentation of the various protocol and
database structure changes, and a small tweak to the test suite to make
sure the whole thing stands a chance of working.