Add support for opening an existing PTY

Virtual machine monitor programs (e.g. QEMU, Cloud Hypervisor) expose
guest consoles as PTYs.  With this patch, foot can access these guest
consoles.

Usually, the program used for accessing these PTYs is screen, but
screen is barely developed, doesn't support resizing, and has a bunch
of other unrelated stuff going on.  It would be nice to have a
terminal emulator that properly supported opening an existing PTY.
The VMM controls the master end of the PTY, so to the other end (in
this case foot), it just behaves like any application running in a
directly-opened PTY, and all that's needed is to change foot's code to
support opening an existing PTY rather than creating one.

Co-authored-by: tanto <tanto@ccc.ac>

server.c

@@ -332,7 +332,7 @@ fdm_client(struct fdm *fdm, int fd, int events, void *data)
     instance->terminal = term_init(
         conf != NULL ? conf : server->conf,
         server->fdm, server->reaper, server->wayl, "footclient", cwd, token,
-        cdata.argc, argv, (const char *const *)envp,
+        NULL, cdata.argc, argv, (const char *const *)envp,
         &term_shutdown_handler, instance);
 
     if (instance->terminal == NULL) {