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/putty.c

@@ -0,0 +1,92 @@
+/*
+ * Unix PuTTY main program.
+ */
+
+#include <stdio.h>
+#include <ctype.h>
+#include <stdlib.h>
+#include <assert.h>
+#include <unistd.h>
+#include <gtk/gtk.h>
+#include <gdk/gdk.h>
+
+#define MAY_REFER_TO_GTK_IN_HEADERS
+
+#include "putty.h"
+#include "ssh.h"
+#include "storage.h"
+
+#include "gtkcompat.h"
+
+/*
+ * Stubs to avoid uxpty.c needing to be linked in.
+ */
+const bool use_pty_argv = false;
+char **pty_argv;                       /* never used */
+char *pty_osx_envrestore_prefix;
+
+/*
+ * Clean up and exit.
+ */
+void cleanup_exit(int code)
+{
+    /*
+     * Clean up.
+     */
+    sk_cleanup();
+    random_save_seed();
+    exit(code);
+}
+
+const struct BackendVtable *select_backend(Conf *conf)
+{
+    const struct BackendVtable *vt =
+        backend_vt_from_proto(conf_get_int(conf, CONF_protocol));
+    assert(vt != NULL);
+    return vt;
+}
+
+void initial_config_box(Conf *conf, post_dialog_fn_t after, void *afterctx)
+{
+    char *title = dupcat(appname, " Configuration");
+    create_config_box(title, conf, false, 0, after, afterctx);
+    sfree(title);
+}
+
+const bool use_event_log = true, new_session = true, saved_sessions = true;
+const bool dup_check_launchable = true;
+
+/*
+ * X11-forwarding-related things suitable for Gtk app.
+ */
+
+char *platform_get_x_display(void) {
+    const char *display;
+    /* Try to take account of --display and what have you. */
+    if (!(display = gdk_get_display()))
+        /* fall back to traditional method */
+        display = getenv("DISPLAY");
+    return dupstr(display);
+}
+
+const bool share_can_be_downstream = true;
+const bool share_can_be_upstream = true;
+
+const unsigned cmdline_tooltype =
+    TOOLTYPE_HOST_ARG |
+    TOOLTYPE_PORT_ARG |
+    TOOLTYPE_NO_VERBOSE_OPTION;
+
+void setup(bool single)
+{
+    sk_init();
+    settings_set_default_protocol(be_default_protocol);
+    /* Find the appropriate default port. */
+    {
+        const struct BackendVtable *vt =
+            backend_vt_from_proto(be_default_protocol);
+        settings_set_default_port(0); /* illegal */
+        if (vt)
+            settings_set_default_port(vt->default_port);
+    }
+}