Set IOPL in the input thread on Solaris
As discussed in https://lists.x.org/archives/xorg-devel/2019-January/057899.html this fixes the X server to work on a Solaris guest inside VMWare by ensuring the input thread has IOPL set, since Solaris tracks IOPL per thread and new threads don't automatically inherit a raised IOPL.
- stops trying to track IOPL status in a global boolean on Solaris, since it's not a global state for the process
- adds a new ddxInputThread call from os layer into ddx layer, and a new xf86OSInputThreadInit call from common layer into os-support layer
- uses those calls to ensure the IOPL is set when initializing the input thread on Solaris