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.

support/sysvinit/Makefile

@@ -1,8 +0,0 @@
-MK=	../../mk
-include ${MK}/os.mk
-
-DIR=	${DATADIR}/support/sysvinit
-INC=	halt.sh inittab README.md
-
-
-include ${MK}/scripts.mk