Factor out Unix Pageant's socket creation.

The code in Pageant that sets up the Unix socket and its containing
directory now lives in a separate file, uxagentsock.c, where it will
also be callable from the upcoming new SSH server when it wants to
create a similar socket for agent forwarding.

While I'm at it, I've also added a feature to create a watchdog
subprocess that will try to clean up the socket and directory once
Pageant itself terminates, in the hope of leaving less cruft lying
around /tmp.

ssh.h

@@ -344,6 +344,9 @@ char *portfwdmgr_connect(PortFwdManager *mgr, Channel **chan_ret,
                          char *hostname, int port, SshChannel *c,
                          int addressfamily);
 
+Socket *platform_make_agent_socket(Plug *plug, const char *dirprefix,
+                                   char **error, char **name);
+
 LogContext *ssh_get_logctx(Ssh *ssh);
 
 /* Communications back to ssh.c from connection layers */