Merge scandir(3) interface update to stable/8.

Merge scandir(3) interface update to stable/8.

MFC r201512:
Modernize scandir(3) and alphasort(3) interfaces according to the IEEE
Std 1003.1-2008.

MFC r201602:
Move scandir(3) and alphasort(3) into XSI namespace.

MFC r201604:
Use thunks to adapt alphasort-like interface to the comparision function
required by qsort() and qsort_r().

MFC r202556 (by ache):
Use strcoll() in opendir() and alphasort(). Remove some comments.

MFC r202572 (by ache):
Revert to using strcmp() for opendir().

MFC r202677 (by ache):
Style.

MFC r202679 (by ache):
Style: rename internal function to opendir_compar().

MFC r202691 (by ache):
For alphasort(3) add reference to strcoll(3).

MFC r202693 (by ache):
Style: reword comment.

git-svn-id: svn://svn.freebsd.org/base/stable/8@202921 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f