migrate fully to meson build system

- drop old build system
- move shared include and source files to common directory
- drop "rc-" prefix from shared include and source files
- move executable-specific code to individual directories under src
- adjust top-level .gitignore file for new build system

This closes #489.

sh/.gitignore

@@ -1,7 +0,0 @@
-functions.sh
-gendepends.sh
-openrc-run.sh
-cgroup-release-agent.sh
-init.sh
-init-early.sh
-binfmt.sh

sh/Makefile

@@ -1,33 +0,0 @@
-DIR=	${LIBEXECDIR}/sh
-SRCS=	init.sh.in functions.sh.in gendepends.sh.in \
-	openrc-run.sh.in ${SRCS-${OS}}
-INC=	rc-mount.sh functions.sh rc-functions.sh runit.sh s6.sh \
-		start-stop-daemon.sh supervise-daemon.sh ${INC-${OS}}
-BIN=	gendepends.sh init.sh openrc-run.sh ${BIN-${OS}}
-
-INSTALLAFTER=	_installafter
-
-MK=	../mk
-include ${MK}/os.mk
-
-SRCS-FreeBSD=
-BIN-FreeBSD=
-
-SRCS-Linux=	binfmt.sh.in cgroup-release-agent.sh.in init-early.sh.in
-BIN-Linux=	binfmt.sh cgroup-release-agent.sh init-early.sh
-INC-Linux= rc-cgroup.sh
-
-SRCS-NetBSD=
-BIN-NetBSD=
-
-include ${MK}/scripts.mk
-
-%.sh: %.sh${SFX}
-	${SED} ${SED_REPLACE} ${SED_EXTRA} $< > $@
-
-_installafter:
-	${INSTALL} -d ${DESTDIR}/${INITDIR}
-	@# Put functions.sh into init for backwards compat
-	ln -snf ${LIBEXECDIR}/sh/functions.sh ${DESTDIR}/${INITDIR} || exit $$?
-
-check test::