Add $SYSTEMD_INVOKED_AS

This allows multi-call binaries to be easily invoked with a different
name. After installation, the name is set by creating a symlink. But in
build directories, we don't create the symlinks. (There are also other
ways to achieve the same thing, e.g. zsh supports $ARGV0, and exec -a
can be used, but those are either non-portable or are more complicated
to use.)  The primary use-case for me is to test --help output for
multicall binaries.

Also reorder the help for env vars to group the more generic ones near
the top.

This was initially proposed in https://github.com/systemd/systemd/pull/24054,
but there were some comments about the implementation. I had a branch
with the patch, but I don't think I ever actually submitted it as a
pull request.

src/shared/help-util.c

@@ -1,8 +1,11 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
+#include <stdlib.h>
+
 #include "alloc-util.h"
 #include "ansi-color.h"
 #include "help-util.h"
+#include "path-util.h"
 #include "pretty-print.h"
 
 /* These are helpers for putting together --help texts in a uniform way with a common output style. Each
@@ -25,12 +28,16 @@
  *   options.[ch] APIs. */
 
 void help_cmdline(const char *arguments) {
+        const char *progname =
+                last_path_component(secure_getenv("SYSTEMD_INVOKED_AS"))
+                ?: program_invocation_short_name;
+
         assert(arguments);
 
         printf("%s>%s %s %s\n",
                ansi_grey(),
                ansi_normal(),
-               program_invocation_short_name,
+               progname,
                arguments);
 }