Sync OpenBSD patchset 986:

Support "jump to" like vi in copy mode using t and T keys. Also add x
and X for delete in edit mode.

From Ben Boeckel, thanks.

tmux.h

@@ -481,6 +481,8 @@ enum mode_key_cmd {
 	MODEKEYCOPY_JUMPAGAIN,
 	MODEKEYCOPY_JUMPREVERSE,
 	MODEKEYCOPY_JUMPBACK,
+	MODEKEYCOPY_JUMPTO,
+	MODEKEYCOPY_JUMPTOBACK,
 	MODEKEYCOPY_LEFT,
 	MODEKEYCOPY_MIDDLELINE,
 	MODEKEYCOPY_NEXTPAGE,