manager: if called with compat telinit interface, tell users to update

In https://bugzilla.redhat.com/show_bug.cgi?id=2479961 a user
reported that they are confused that 'init 6' and such commands
do not work anymore. We removed support for the whole interface,
but it's likely that such commands persist in various scripts
and finger memories. Let's give a helpful hint that this inteface
is gone and what to use instead.

src/core/main.c

@@ -89,6 +89,7 @@
 #include "parse-util.h"
 #include "path-util.h"
 #include "pidfd-util.h"
+#include "pretty-print.h"
 #include "proc-cmdline.h"
 #include "process-util.h"
 #include "random-util.h"
@@ -1004,6 +1005,31 @@ static void set_manager_settings(Manager *m) {
         m->restrict_filesystem_access = arg_restrict_filesystem_access;
 }
 
+static int redirect_telinit(char *argv[], char **args) {
+        /* Check if we are invoked through the legacy interface, where init would be symlinked as telinit
+         * and allow users to call 'init 0' and such. If we detect such use, tell the user that this is not
+         * supported anymore. */
+
+        if (getpid_cached() == 1 || !invoked_as(argv, "init"))
+                return 0;
+
+        /* Check if the user specified one of the telinit commands in args. */
+        if (!strv_overlap(args,
+                          STRV_MAKE("0", "1", "2", "3", "4", "5", "6",
+                                    "s", "S", "q", "Q", "u", "U")))
+                return 0;
+
+        _cleanup_free_ char *a = NULL, *b = NULL, *c = NULL;
+        (void) terminal_urlify_man_full("shutdown", "8", /* suffix= */ NULL, &a);
+        (void) terminal_urlify_man_full("reboot", "8", /* suffix= */ NULL, &b);
+        (void) terminal_urlify_man_full("systemctl", "1", /* suffix= */ NULL, &c);
+
+        return log_error_errno(SYNTHETIC_ERRNO(EOPNOTSUPP),
+                               "Program 'systemd' called as 'init' with a legacy telinit command.\n"
+                               "Call %s, %s, or %s instead.",
+                               strnull(a), strnull(b), strnull(c));
+}
+
 static int parse_argv(int argc, char *argv[]) {
         bool user_arg_seen = false;
         int r;
@@ -1234,6 +1260,10 @@ static int parse_argv(int argc, char *argv[]) {
                         return 0;
                 }
 
+        r = redirect_telinit(argv, option_parser_get_args(&opts));
+        if (r < 0)
+                return r;
+
         if (option_parser_get_n_args(&opts) > 0 && getpid_cached() != 1)
                 /* Hmm, when we aren't run as init system let's complain about excess arguments */
                 return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Excess arguments.");