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.

etc/.gitignore

@@ -1,2 +0,0 @@
-rc
-rc.shutdown

etc/Makefile

@@ -1,23 +0,0 @@
-DIR=	${SYSCONFDIR}
-SRCS=	rc.in rc.shutdown.in
-BIN=	${BIN-${OS}}
-CONF=	rc.conf ${BIN-${OS}}
-
-MK=	../mk
-include ${MK}/os.mk
-
-SED_EXTRA-FreeBSD=	-e 's:@TERM@:cons25:g'
-BIN-FreeBSD=		rc rc.shutdown rc.devd
-CONF-FreeBSD=		devd.conf
-
-SED_EXTRA-Linux=	-e 's:@TERM@:wsvt25:g'
-BIN-Linux=
-CONF-Linux=
-
-SED_EXTRA-NetBSD=	-e 's:@TERM@:wsvt25:g'
-BIN-NetBSD=		rc rc.shutdown
-CONF-NetBSD=
-
-SED_EXTRA=	${SED_EXTRA-${OS}}
-
-include ${MK}/scripts.mk