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/config-unix.c

@@ -0,0 +1,70 @@
+/*
+ * uxcfg.c - the Unix-specific parts of the PuTTY configuration
+ * box.
+ */
+
+#include <assert.h>
+#include <stdlib.h>
+
+#include "putty.h"
+#include "dialog.h"
+#include "storage.h"
+
+void unix_setup_config_box(struct controlbox *b, bool midsession, int protocol)
+{
+    struct controlset *s;
+    union control *c;
+
+    /*
+     * The Conf structure contains two Unix-specific elements which
+     * are not configured in here: stamp_utmp and login_shell. This
+     * is because pterm does not put up a configuration box right at
+     * the start, which is the only time when these elements would
+     * be useful to configure.
+     */
+
+    /*
+     * On Unix, we don't have a drop-down list for the printer
+     * control.
+     */
+    s = ctrl_getset(b, "Terminal", "printing", "Remote-controlled printing");
+    assert(s->ncontrols == 1 && s->ctrls[0]->generic.type == CTRL_EDITBOX);
+    s->ctrls[0]->editbox.has_list = false;
+
+    /*
+     * Unix supports a local-command proxy. This also means we must
+     * adjust the text on the `Telnet command' control.
+     */
+    if (!midsession) {
+        int i;
+        s = ctrl_getset(b, "Connection/Proxy", "basics", NULL);
+        for (i = 0; i < s->ncontrols; i++) {
+            c = s->ctrls[i];
+            if (c->generic.type == CTRL_RADIO &&
+                c->generic.context.i == CONF_proxy_type) {
+                assert(c->generic.handler == conf_radiobutton_handler);
+                c->radio.nbuttons++;
+                c->radio.buttons =
+                    sresize(c->radio.buttons, c->radio.nbuttons, char *);
+                c->radio.buttons[c->radio.nbuttons-1] =
+                    dupstr("Local");
+                c->radio.buttondata =
+                    sresize(c->radio.buttondata, c->radio.nbuttons, intorptr);
+                c->radio.buttondata[c->radio.nbuttons-1] = I(PROXY_CMD);
+                break;
+            }
+        }
+
+        for (i = 0; i < s->ncontrols; i++) {
+            c = s->ctrls[i];
+            if (c->generic.type == CTRL_EDITBOX &&
+                c->generic.context.i == CONF_proxy_telnet_command) {
+                assert(c->generic.handler == conf_editbox_handler);
+                sfree(c->generic.label);
+                c->generic.label = dupstr("Telnet command, or local"
+                                          " proxy command");
+                break;
+            }
+        }
+    }
+}