warn users of deprecated functions

• Warn users of command-line arguments --unix and --xen that support for
  these will soon be dropped. Instead, use -t unix and -t xen respectively.
  (see mirage/mirage-www#475 (comment))
  (#561, by @yomimono)
• Warn users of functions add_to_opam_packages p and
  add_to_ocamlfind_libraries l that support for these will soon be dropped.
  Instead, use register ~libraries:l and register:~packages:p
  respectively. (#561, by @yomimono).