libposix: deep refactor; add sys/posixly command

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

qa/lib/newlib/001-hello.c

@@ -0,0 +1,7 @@
+#include <stdio.h>
+int main()
+{
+	fprintf(stdout, "hello, stdout\n");
+	fprintf(stderr, "hello, stderr\n");
+	return 0;
+}