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.h

@@ -486,6 +486,7 @@ void exec_context_revert_tty(ExecContext *c);
 
 int exec_context_get_clean_directories(ExecContext *c, char **prefix, ExecCleanMask mask, char ***ret);
 int exec_context_get_clean_mask(ExecContext *c, ExecCleanMask *ret);
+int exec_context_add_default_dependencies(Unit *u, const ExecContext *c);
 
 void exec_status_start(ExecStatus *s, pid_t pid);
 void exec_status_exit(ExecStatus *s, const ExecContext *context, pid_t pid, int code, int status);