Log identifying information for the other end of connections.

When anyone connects to a PuTTY tool's listening socket - whether it's
a user of a local->remote port forwarding, a connection-sharing
downstream or a client of Pageant - we'd like to log as much
information as we can find out about where the connection came from.

To that end, I've implemented a function sk_peer_info() in the socket
abstraction, which returns a freeform text string as best it can (or
NULL, if it can't get anything at all) describing the thing at the
other end of the connection. For TCP connections, this is done using
getpeername() to get an IP address and port in the obvious way; for
Unix-domain sockets, we attempt SO_PEERCRED (conditionalised on some
moderately hairy autoconfery) to get the pid and owner of the peer. I
haven't implemented anything for Windows named pipes, but I will if I
hear of anything useful.

Recipe

@@ -230,8 +230,8 @@ SFTP     = sftp int64 logging
 MISC     = timing callback misc version settings tree234 proxy conf
 WINMISC  = MISC winstore winnet winhandl cmdline windefs winmisc winproxy
          + wintime winhsock errsock
-UXMISC   = MISC uxstore uxsel uxnet cmdline uxmisc uxproxy time
-OSXMISC  = MISC uxstore uxsel osxsel uxnet uxmisc uxproxy time
+UXMISC   = MISC uxstore uxsel uxnet uxpeer cmdline uxmisc uxproxy time
+OSXMISC  = MISC uxstore uxsel osxsel uxnet uxpeer uxmisc uxproxy time
 
 # import.c and dependencies, for PuTTYgen-like utilities that have to
 # load foreign key files.