basic: move a bunch of cmdline-related funcs to new argv-util.c+h

I wanted to move saved_arg[cv] to process-util.c+h, but this causes problems:
process-util.h includes format-util.h which includes net/if.h, which conflicts
with linux/if.h. So we can't include process-util.h in some files.

But process-util.c is very long anyway, so it seems nice to create a new file.
rename_process(), invoked_as(), invoked_by_systemd(), and argv_looks_like_help()
which lived in process-util.c refer to saved_argc and saved_argv, so it seems
reasonable to move them to the new file too.

util.c is now empty, so it is removed. util.h remains.

src/basic/meson.build

@@ -9,6 +9,8 @@ basic_sources = files(
         'alloc-util.h',
         'architecture.c',
         'architecture.h',
+        'argv-util.c',
+        'argv-util.h',
         'arphrd-util.c',
         'arphrd-util.h',
         'async.c',
@@ -256,7 +258,6 @@ basic_sources = files(
         'user-util.h',
         'utf8.c',
         'utf8.h',
-        'util.c',
         'util.h',
         'virt.c',
         'virt.h',