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.

zsh-completion/Makefile

@@ -1,10 +0,0 @@
-DIR=	${ZSHCOMPDIR}
-CONF=	_openrc \
-		_rc-service \
-		_rc-status \
-		_rc-update \
-
-MK=	../mk
-include ${MK}/os.mk
-
-include ${MK}/scripts.mk