libposix: deep refactor; add sys/posixly command

With these changes, libposix (and newlib) can run MirBSD Korn Shell.

qa/lib/newlib/002-atexit.c

@@ -0,0 +1,27 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+void
+bye(void)
+{
+	printf("That was all, folks\n");
+	exit(0);
+}
+
+int
+main(void)
+{
+//	long a;
+	int i;
+
+//	a = sysconf(_SC_ATEXIT_MAX);
+//	printf("ATEXIT_MAX = %ld\n", a);
+
+	i = atexit(bye);
+	if (i != 0) {
+		fprintf(stderr, "cannot set exit function\n");
+		return 1;
+	}
+	return 2;
+}