Get rid of lots of implicit pointer types.

All the main backend structures - Ssh, Telnet, Pty, Serial etc - now
describe structure types themselves rather than pointers to them. The
same goes for the codebase-wide trait types Socket and Plug, and the
supporting types SockAddr and Pinger.

All those things that were typedefed as pointers are older types; the
newer ones have the explicit * at the point of use, because that's
what I now seem to be preferring. But whichever one of those is
better, inconsistently using a mixture of the two styles is worse, so
let's make everything consistent.

A few types are still implicitly pointers, such as Bignum and some of
the GSSAPI types; generally this is either because they have to be
void *, or because they're typedefed differently on different
platforms and aren't always pointers at all. Can't be helped. But I've
got rid of the main ones, at least.

unix/unix.h

@@ -314,7 +314,7 @@ int init_ucs(struct unicode_data *ucsdata, char *line_codepage,
 /*
  * Spare function exported directly from uxnet.c.
  */
-void *sk_getxdmdata(Socket sock, int *lenp);
+void *sk_getxdmdata(Socket *sock, int *lenp);
 
 /*
  * General helpful Unix stuff: more helpful version of the FD_SET