packages: Compress in parallel

The FreeBSD-base pkg repository is currrently dominated by a handful of
large packages: Out of a compressed repository size of ~960 MB, about
160 MB is taken up by the FreeBSD-src package, 128 MB (on amd64) is
used by the FreeBSD-kernel-generic-dbg package, and 91 MB is used by
the FreeBSD-src-sys package.  Consequently, running 'make packages -jN'
provides less benefit than one might hope, as most of the packages
finish building quickly, ultimately leaving the FreeBSD-src package
building by itself for a couple minutes while all the other CPUs are
idle.

Pass -T${PKG_CTHREADS} to the 'pkg create' commands (with a default
of -T0) in order to instruct pkg's zstd compression to use multiple
threads.

Testing on an EC2 r7i.48xlarge instance with -j192, this reduces the
time taken by 'make packages' from 6m17s to 1m39s; package creation
time itself (excluding the initial process of installing into world
and kernel staging directories) dropped from 5m37s to 59s.

Reviewed by:	ivy
MFC after:	3 days
Sponsored by:	https://www.patreon.com/cperciva
Differential Revision:	https://reviews.freebsd.org/D53053

Author: cperciva

Makefile.inc1

@@ -1964,6 +1964,7 @@ REPODIR?=	${OBJROOT}repo
 PKG_FORMAT?=	tzst
 PKG_LEVEL?=	-1
 PKG_CLEVEL?=	${"${PKG_FORMAT:Mtar}" != "":?:-l ${PKG_LEVEL}}
+PKG_CTHREADS?=	0
 PKG_REPO_SIGNING_KEY?=	# empty
 PKG_OUTPUT_DIR?=	${PKG_VERSION}
 PKG_ABI_FILE?=	${WSTAGEDIR}/usr/bin/uname
@@ -2144,7 +2145,7 @@ create-source-src-package: _pkgbootstrap .PHONY
 		${SSTAGEDIR}/src.ucl
 	${PKG_CMD} -o ABI=${PKG_ABI} \
 		-o OSVERSION="${SRCRELDATE}" \
-		create -f ${PKG_FORMAT} ${PKG_CLEVEL} \
+		create -f ${PKG_FORMAT} ${PKG_CLEVEL} -T${PKG_CTHREADS} \
 		-M ${SSTAGEDIR}/src.ucl \
 		-p ${SSTAGEDIR}/src.plist \
 		-r ${SRCDIR} \
@@ -2170,7 +2171,7 @@ create-source-src-sys-package: _pkgbootstrap .PHONY
 		${SSTAGEDIR}/src-sys.ucl
 	${PKG_CMD} -o ABI=${PKG_ABI} \
 		-o OSVERSION="${SRCRELDATE}" \
-		create -f ${PKG_FORMAT} ${PKG_CLEVEL} \
+		create -f ${PKG_FORMAT} ${PKG_CLEVEL} -T${PKG_CTHREADS} \
 		-M ${SSTAGEDIR}/src-sys.ucl \
 		-p ${SSTAGEDIR}/src-sys.plist \
 		-r ${SRCDIR} \
@@ -2210,7 +2211,7 @@ create-world-package-${pkgname}: .PHONY
 		' ${WSTAGEDIR}/${pkgname}.ucl
 	${PKG_CMD} -o ABI=${PKG_ABI} -o ALLOW_BASE_SHLIBS=yes \
 		-o OSVERSION="${SRCRELDATE}" \
-		create -f ${PKG_FORMAT} ${PKG_CLEVEL} \
+		create -f ${PKG_FORMAT} ${PKG_CLEVEL} -T${PKG_CTHREADS} \
 		-M ${WSTAGEDIR}/${pkgname}.ucl \
 		-p ${WSTAGEDIR}/${pkgname}.plist \
 		-r ${WSTAGEDIR} \
@@ -2229,7 +2230,7 @@ create-sets-packages: .PHONY
 	@for manifest in ${WSTAGEDIR}/set-*.ucl; do \
 		echo "--> Processing manifest: $$manifest"; \
 		${PKG_CMD} -o ABI=${PKG_ABI} -o OSVERSION="${SRCRELDATE}" \
-			create -f ${PKG_FORMAT} ${PKG_CLEVEL} \
+			create -f ${PKG_FORMAT} ${PKG_CLEVEL} -T${PKG_CTHREADS} \
 			-M $$manifest \
 			-o "${REPODIR}/${PKG_ABI}/${PKG_OUTPUT_DIR}" \
 			|| exit 1; \
@@ -2259,7 +2260,7 @@ create-dtb-package:	.PHONY
 		${KSTAGEDIR}/${DISTDIR}/dtb.ucl ; \
 	${PKG_CMD} -o ABI=${PKG_ABI} -o ALLOW_BASE_SHLIBS=yes \
 		-o OSVERSION="${SRCRELDATE}" \
-		create -f ${PKG_FORMAT} ${PKG_CLEVEL} \
+		create -f ${PKG_FORMAT} ${PKG_CLEVEL} -T${PKG_CTHREADS} \
 		-M ${KSTAGEDIR}/${DISTDIR}/dtb.ucl \
 		-p ${KSTAGEDIR}/${DISTDIR}/dtb.plist \
 		-r ${KSTAGEDIR}/${DISTDIR} \
@@ -2296,7 +2297,7 @@ create-kernel-packages-flavor${flavor:C,^""$,${_default_flavor},}: _pkgbootstrap
 		${KSTAGEDIR}/${DISTDIR}/kernel.${INSTALLKERNEL}${flavor}.ucl ; \
 	${PKG_CMD} -o ABI=${PKG_ABI} -o ALLOW_BASE_SHLIBS=yes \
 		-o OSVERSION="${SRCRELDATE}" \
-		create -f ${PKG_FORMAT} ${PKG_CLEVEL} \
+		create -f ${PKG_FORMAT} ${PKG_CLEVEL} -T${PKG_CTHREADS} \
 		-M ${KSTAGEDIR}/${DISTDIR}/kernel.${INSTALLKERNEL}${flavor}.ucl \
 		-p ${KSTAGEDIR}/${DISTDIR}/kernel.${INSTALLKERNEL}${flavor}.plist \
 		-r ${KSTAGEDIR}/${DISTDIR} \
@@ -2339,7 +2340,7 @@ create-kernel-packages-extra-flavor${flavor:C,^""$,${_default_flavor},}-${_kerne
 		${KSTAGEDIR}/kernel.${_kernel}/kernel.${_kernel}${flavor}.ucl ; \
 	${PKG_CMD} -o ABI=${PKG_ABI} -o ALLOW_BASE_SHLIBS=yes \
 		-o OSVERSION="${SRCRELDATE}" \
-		create -f ${PKG_FORMAT} ${PKG_CLEVEL} \
+		create -f ${PKG_FORMAT} ${PKG_CLEVEL} -T${PKG_CTHREADS} \
 		-M ${KSTAGEDIR}/kernel.${_kernel}/kernel.${_kernel}${flavor}.ucl \
 		-p ${KSTAGEDIR}/kernel.${_kernel}/kernel.${_kernel}${flavor}.plist \
 		-r ${KSTAGEDIR}/kernel.${_kernel} \