<features.h> declares _SORTIX_SOURCE if no conflicting macros are declared, such as _GNU_SOURCE. Fixed g++ automatically declaring _GNU_SOURCE, but Sortix isn't GNU. Replaced SORTIX_UNIMPLEMENTED macro with __SORTIX_SHOW_UNIMPLEMENTED.
opendir(3). Also added non-standard dregister(3), dunregister(3), dclearerr(3), derror(3), deof(3), dnewdir(3), dcloseall(3).