core: inherit TERM from PID 1 for all services started on /dev/console

This way, invoking nspawn from a shell in the best case inherits the TERM
setting all the way down into the login shell spawned in the container.

Fixes: #3697

src/basic/terminal-util.c

@@ -785,7 +785,7 @@ bool tty_is_vc_resolve(const char *tty) {
 }
 
 const char *default_term_for_tty(const char *tty) {
-        return tty && tty_is_vc_resolve(tty) ? "TERM=linux" : "TERM=vt220";
+        return tty && tty_is_vc_resolve(tty) ? "linux" : "vt220";
 }
 
 int fd_columns(int fd) {