Allow different peer associations to use different private keys.

Allow different peer associations to use different private keys.

Add a `peertag' slot to the `peerspec' structure stating which private
key to use; a null pointer means to use the default `tag_priv'.  Add a
`p_privtag' function to retrieve the private key tag to use.

The ADD command now has a `-priv' option which sets this slot
appropriately.  The `connect' service fetches a `priv' key. from the
database to set this option.