subshell: tell fish that we actually support the kitty keyboard proto…

…col now

__mc_csi_u was added so that fish shell knows when it's safe to enable the
kitty keyboard protocol.

As seen in the parent commit, it wasn't quite safe; so the upcoming patch
release of fish will disable this protocol again -- until fish finds this
commit, so add a new version marker.

While at it, use a less misleading name.  Dropping the old name will mean that
fish < 4.0.1 will no longer enable the kitty keyboard protocol (and be safe
from the hang on numlock). I guess we could avoid that but I don't think this
is not a big loss, especially because since fish 4.0.1 will enable it again.

src/subshell/common.c

@@ -465,7 +465,7 @@ init_subshell_child (const char *pty_name)
 
     case SHELL_FISH:
         execl (mc_global.shell->path, mc_global.shell->path, "--init-command",
-               "set --global __mc_csi_u 1", (char *) NULL);
+               "set --global __mc_kitty_keyboard 1", (char *) NULL);
         break;
 
     case SHELL_ASH_BUSYBOX: