Add Haiku support

[pulkomandy]

This is a rebase of #26 on the current sources with the following notes:

• There were merge conflicts due to adding Illumos support. Nothing really complicated to fix.
• Since this was initially developped, Haiku did get an implementation for cfsetspeed, so use that instead of the reimplementation.
• Confirmed building and passing tests on a current Haiku nightly build (as Haiku should be quite close to shipping a new beta release, I feel no need to support the soon deprecated beta 5).