We already have <tools/prex.h> and <tools/postx.h>, so make those be
sufficient instead. Bin another local vcl header vcl/inc/unx/svunx.h that just
included those prex.h and postx.h. Adapt includers accordingly.

Change-Id: I6638b3260fd3da45478fcc216b41f8c8a539f0d7