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.

src/poweroff/meson.build

@@ -0,0 +1,7 @@
+if os == 'Linux' and get_option('sysvinit')
+  configure_file(input : 'poweroff.in',
+    output : '@BASENAME@',
+    configuration : script_conf_data,
+    install_dir: sbindir,
+    install_mode: 'rwxr-xr-x')
+endif

src/poweroff/poweroff.in

@@ -0,0 +1,28 @@
+#!/bin/sh
+
+option_arg=
+poweroff_arg=
+while getopts :nwdfiph opt; do
+	case "$opt" in
+		n) ;;
+		w) poweroff_arg=--write-only ;;
+		d) option_arg=--no-write ;;
+		f) ;;
+		i) ;;
+		[?]) printf "%s\n" "${0##*/}: invalid command line option" >&2
+		exit 1
+		;;
+	esac
+done
+shift $((OPTIND-1))
+
+if [ -z "${poweroff_arg}" ]; then
+	poweroff_arg=--poweroff
+fi
+
+script_args="$@"
+if [ -z "${script_args}" ]; then
+	script_args=now
+fi
+
+exec @SBINDIR@/openrc-shutdown ${option_arg} ${poweroff_arg} "${script_args}"