crash-handler: Call vhangup on /dev/console before spawning crash shell

When pid 1 crashes, the getty unit for the console will happily keep
running which means we end up with two shells competing for the same
tty. Let's call vhangup on /dev/console to kill every other process
attached to the console before we spawn the crash shell. The getty
units have Restart=always but lucky for us, pid 1 just crashed in fire
and flames so it isn't actually able to restart the getty unit.

src/basic/terminal-util.c

@@ -434,6 +434,18 @@ int terminal_vhangup_fd(int fd) {
         return RET_NERRNO(ioctl(fd, TIOCVHANGUP));
 }
 
+int terminal_vhangup(const char *tty) {
+        _cleanup_close_ int fd = -EBADF;
+
+        assert(tty);
+
+        fd = open_terminal(tty, O_RDWR|O_NOCTTY|O_CLOEXEC);
+        if (fd < 0)
+                return fd;
+
+        return terminal_vhangup_fd(fd);
+}
+
 int vt_disallocate(const char *tty_path) {
         assert(tty_path);