etc/build.usage: adds -P [jobs].
subr/build_args.subr: adds `optarg' arg type & ARG_PARALLEL:optarg:-P:auto.
subr/build_init.subr: default ${ARG_PARALLEL} ([jobs]) to ${DEFAULT_BUILD_CPUS}.
subr/ex_pkg_dispatch.subr: honour ${ARG_PARALLEL}, defaulting to no parallelisation.