Disaggregate libraries in OS

• Fix uninstall target for Unix.
• Remove tuntap stubs from Unix module; they are in ocaml-tuntap now.
• Move OS.Clock out to a toplevel Clock module (the mirage-clock package).
• Move OS.Io_page out to a toplevel Io_page module (the io-page package).
• Update library dependencies to reduce them based on new functionality.
• Install library as mirage-xen or mirage-unix that can coexist.
• Suppress dietlibc linker warnings for sscanf/sprintf.