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/shell_var/meson.build

@@ -0,0 +1,4 @@
+executable('shell_var',
+  ['shell_var.c'],
+  install: true,
+  install_dir: rc_bindir)

src/shell_var/shell_var.c

@@ -0,0 +1,41 @@
+/*
+ * Copyright (c) 2007-2015 The OpenRC Authors.
+ * See the Authors file at the top-level directory of this distribution and
+ * https://github.com/OpenRC/openrc/blob/HEAD/AUTHORS
+ *
+ * This file is part of OpenRC. It is subject to the license terms in
+ * the LICENSE file found in the top-level directory of this
+ * distribution and at https://github.com/OpenRC/openrc/blob/HEAD/LICENSE
+ * This file may not be copied, modified, propagated, or distributed
+ *    except according to the terms contained in the LICENSE file.
+ */
+
+#include <errno.h>
+#include <ctype.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <time.h>
+#include <unistd.h>
+
+int main(int argc, char **argv)
+{
+	int i;
+	char *p;
+	int c;
+
+	for (i = 1; i < argc; i++) {
+		p = argv[i];
+		if (i != 1)
+			putchar(' ');
+		while (*p) {
+			c = (unsigned char)*p++;
+			if (!isalnum(c))
+				c = '_';
+			putchar(c);
+		}
+	}
+	putchar('\n');
+	return EXIT_SUCCESS;
+}