Fixed inconsistent use of S_IFDIR et al

* If S_ISDIR, S_ISREG not defined by compiler, czmq_prelude.h now
  defines them.

* zfile_is_directory and zfile_is_regular now use these macros.

* CZMQ now uses POSIX S_IRUSR and S_IWUSR instead of sometimes
  S_IREAD and S_IWRITE, and defines the S_IRUSR and S_IWUSR macros
  if they're needed.

Fixes #504