Encapsulate use of nice()

- Instead of platform-specific #ifdefs in otherwise platform-agnostic
  code, encapsulate the call in sysinfo.h/.cpp.
- Make it fully CMake's responsibility to determine whether nice() is
  going to be used.  This could probably be simplified further (even to
  the point of using HAVE_NICE directly), but since I'm not sure how all
  the special cases work, left them as they were, but with TODO
  comments.
- Remove some unused platform-specific #includes.

Change-Id: I9f1172ba2c02bb042574ad5d2b9ccbf5d31a3d6f