Rename most of the platform source files.

This gets rid of all those annoying 'win', 'ux' and 'gtk' prefixes
which made filenames annoying to type and to tab-complete. Also, as
with my other recent renaming sprees, I've taken the opportunity to
expand and clarify some of the names so that they're not such cryptic
abbreviations.

windows/nohelp.c

@@ -0,0 +1,15 @@
+/*
+ * nohelp.c: implement the has_embedded_chm() function for
+ * applications that have no help file at all, so that misc.c's
+ * buildinfo string knows not to talk meaninglessly about whether the
+ * nonexistent help file is present.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <assert.h>
+
+#include "putty.h"
+
+int has_embedded_chm(void) { return -1; }