basic: fold getopt-defs.h into proc-cmdline.c

It'd be nice if we could not repeat the information about the option
list a second time. But I don't see a nice way to do this, since
(by design) with the macro approach, the macros must be intertwined
with the parse_argv() code. But that code in turn refers to a bunch
of variables, so lifting out the function is not immediately possible.
So I think it's best to keep the existing approach where we provide
a list of options, without additional context, and skip them using
a custom routine.

TODO.md

@@ -3021,6 +3021,9 @@ SPDX-License-Identifier: LGPL-2.1-or-later
 - whenever we receive fds via SCM_RIGHTS make sure none got dropped due to the
   reception limit the kernel silently enforces.
 
+- after option+verb introspection is added, add a test to verify that the
+  list in proc-cmdline.c matches the actual option list in systemd and shutdown.
+
 - write a document explaining how to write correct udev rules. Mention things
   such as:
   1. do not do lists of vid/pid matches, use hwdb for that