include: posix: Split dirent.h from unistd.h

From POSIX
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/dirent.h.html



"""
The <dirent.h> header shall define the following type:
DIR

...

also define the structure dirent
"""

etc.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>