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.

unix/no-gtk.c

@@ -0,0 +1,11 @@
+/*
+ * uxnogtk.c: link into non-GUI Unix programs so that they can tell
+ * buildinfo about a lack of GTK.
+ */
+
+#include "putty.h"
+
+char *buildinfo_gtk_version(void)
+{
+    return NULL;
+}