Our logging uses program_invocation_short_name. Without this patch,
logs from forked client may become broken; spuriously truncated or
the short invocation name is not completely shown in the log.
Process renaming happens very seldomly so we are able to afford proper
permission check, i.e. actually check for CAP_SYS_RESOURCE capability
instead of euid.
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.