pid1: order units using TTYVHangup= after vconsole setup

The goal of this change is to delay getty services until after
systemd-vconsole-setup has finished. systemd-vconsole-setup starts loadkeys,
and it seems that when loadkeys is interrupted by the TTY hangup call we do
when starting tty services [1], so that loadkeys starts getting EIO from the
ioctl("/dev/tty1", KDSKBENT) syscall it does.

Fixes #26908.

[1] https://github.com/legionus/kbd/issues/92#issuecomment-1554451788

Initially I wanted to add ordering dependencies to individual units, but
TTYVHangup= can be added to other various external units too. The solution with
an implicit dependency should cover those cases too.

src/core/execute.c

@@ -6870,6 +6870,16 @@ bool exec_context_has_encrypted_credentials(ExecContext *c) {
         return false;
 }
 
+int exec_context_add_default_dependencies(Unit *u, const ExecContext *c) {
+        assert(u);
+        assert(u->default_dependencies);
+
+        if (c && exec_context_needs_term(c))
+                return unit_add_dependency_by_name(u, UNIT_AFTER, SPECIAL_VCONSOLE_SETUP_SERVICE,
+                                                   /* add_reference= */ true, UNIT_DEPENDENCY_DEFAULT);
+        return 0;
+}
+
 void exec_status_start(ExecStatus *s, pid_t pid) {
         assert(s);