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/select-gui.c

@@ -0,0 +1,38 @@
+/*
+ * Implementation of do_select() for winnet.c to use, that uses
+ * WSAAsyncSelect to convert network activity into window messages,
+ * for integration into a GUI event loop.
+ */
+
+#include "putty.h"
+
+static HWND winsel_hwnd = NULL;
+
+void winselgui_set_hwnd(HWND hwnd)
+{
+    winsel_hwnd = hwnd;
+}
+
+void winselgui_clear_hwnd(void)
+{
+    winsel_hwnd = NULL;
+}
+
+const char *do_select(SOCKET skt, bool enable)
+{
+    int msg, events;
+    if (enable) {
+        msg = WM_NETEVENT;
+        events = (FD_CONNECT | FD_READ | FD_WRITE |
+                  FD_OOB | FD_CLOSE | FD_ACCEPT);
+    } else {
+        msg = events = 0;
+    }
+
+    assert(winsel_hwnd);
+
+    if (p_WSAAsyncSelect(skt, winsel_hwnd, msg, events) == SOCKET_ERROR)
+        return winsock_error_string(p_WSAGetLastError());
+
+    return NULL;
+}