Add Socket::Option.

* lib/19/socket.rb:
  Make BasicSocket#getsockopt return a Socket::Option instead
  of a string.
  Make BasicSocket#setsockopt accept a Socket::Option as
  uniq parameter.
  Add class Socket::Option

* rakelib/platform.rake:
  Add linger structure used by Socket::Option.

* spec/tags/19/ruby/library/socket/socket/connect_nonblock_tags.txt:
  Bonus: Socket::Option makes this spec pass!