Use an Option<i64> for r_id instead of a plain i64, and use None instead
of -1 to indicate the id does not exist in the handles.

Use an auto incrementing u64 value as the OsIpcReceiver's id within the
OsIpcReceiverSet in the inprocess and unix modules.

  - Change the type returned by OsIpcReceiverSet select and add to u64,
    to delay overflow.
  - Factor out the implementation of an auto-incrementing id
    inprocess into os::Incrementor, to be used by other platforms.
  - Use the Incrementor in the unix module to decouple the id of the
    OsIpcReceiver from the file descriptor to avoid race conditions due
    to file descriptor reuse.

For unix OSes make sure to close the file descriptor on ChannelClosed.
The file descriptors should be closed in select to avoid leaking fds
after removal from pollfds (servo#96). After e06edbc this is safe and
avoids the previously seen race condition due to file descriptor
reuse.