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/basic/argv-util.c

@@ -29,13 +29,15 @@ void save_argc_argv(int argc, char **argv) {
 }
 
 bool invoked_as(char *argv[], const char *token) {
-        if (!argv || isempty(argv[0]))
+        const char *progname = secure_getenv("SYSTEMD_INVOKED_AS");
+
+        if (!progname && argv)
+                progname = argv[0];
+
+        if (isempty(progname) || isempty(token))
                 return false;
 
-        if (isempty(token))
-                return false;
-
-        return strstr(last_path_component(argv[0]), token);
+        return strstr(last_path_component(progname), token);
 }
 
 bool invoked_by_systemd(void) {