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/process-util.h

@@ -71,7 +71,6 @@ void sigterm_wait(pid_t pid);
 
 int kill_and_sigcont(pid_t pid, int sig);
 
-int rename_process(const char name[]);
 int is_kernel_thread(pid_t pid);
 
 int getenv_for_pid(pid_t pid, const char *field, char **_value);
@@ -191,10 +190,4 @@ int pidfd_get_pid(int fd, pid_t *ret);
 
 int setpriority_closest(int priority);
 
-bool invoked_as(char *argv[], const char *token);
-
-bool invoked_by_systemd(void);
-
 _noreturn_ void freeze(void);
-
-bool argv_looks_like_help(int argc, char **argv);