Reorg of compat code.

Reorg of compat code.

Instead if usual/compat.[ch] and usual/win32.h, the compat code is
now merged with 'theme' modules:

  <usual/base.h>
  <usual/fileutil.h>
  <usual/signal.h>
  <usual/socket.h>
  <usual/string.h>
  <usual/time.h>

New 'theme include' concept mean those include bunch of related
system includes, provide compat funtions for missing system
functionality in addition to providing new APIs.

End result should be that user code does not need have several
'#ifdef HAVE..H' around system includes, instead it can just
include theme include to get all related functionality.