Use sysread and syswrite on Windows instead of read_nonblock and write_n...

Use sysread and syswrite on Windows instead of read_nonblock and write_nonblock in proxy command pipe (#168)
Looks like it does match on cygwin: https://github.com/rubygems/rubygems/blob/master/lib/rubygems.rb#L121-L127

Also, it looks like cygwin does support setting O_NONBLOCK on fds, though it looks like it is historically buggy, so I'm not sure if adding an exclusion for cygwin for Gem.win_platform? to maintain the current functionality or if it should be lumped in with the other windows platforms for safety.