Support for Windows PuTTY connecting straight to a local serial port

in place of making a network connection. This has involved a couple
of minor infrastructure changes:
 - New dlg_label_change() function in the dialog.h interface, which
   alters the label on a control. Only used, at present, to switch
   the Host Name and Port boxes into Serial Line and Speed, which
   means that any platform not implementing serial connections (i.e.
   currently all but Windows) does not need to actually do anything
   in this function. Yet.
 - New small piece of infrastructure: cfg_launchable() determines
   whether a Config structure describes a session ready to be
   launched. This was previously determined by seeing if it had a
   non-empty host name, but it has to check the serial line as well
   so there's a centralised function for it. I haven't gone through
   all front ends and arranged for this function to be used
   everywhere it needs to be; so far I've only checked Windows.
 - Similarly, cfg_dest() returns the destination of a connection
   (host name or serial line) in a text format suitable for putting
   into messages such as `Unable to connect to %s'.

[originally from svn r6815]

windows/winhelp.h

@@ -131,6 +131,12 @@
 #define WINHELP_CTX_ssh_bugs_rsapad2 "ssh.bugs.rsapad2"
 #define WINHELP_CTX_ssh_bugs_pksessid2 "ssh.bugs.pksessid2"
 #define WINHELP_CTX_ssh_bugs_rekey2 "ssh.bugs.rekey2"
+#define WINHELP_CTX_serial_line "serial.line"
+#define WINHELP_CTX_serial_speed "serial.speed"
+#define WINHELP_CTX_serial_databits "serial.databits"
+#define WINHELP_CTX_serial_stopbits "serial.stopbits"
+#define WINHELP_CTX_serial_parity "serial.parity"
+#define WINHELP_CTX_serial_flow "serial.flow"
 
 /* These are used in Windows-specific bits of the frontend.
  * We (ab)use "help context identifiers" (dwContextId) to identify them. */