[hooks] dirent: Wrappers for readdir and readdir_r, hook funcs

struct dirent differs between bionic and glibc, so we need to hook
readdir and readdir_r to translate between the two structs. Also,
all functions in dirent.h that return or take a DIR * also need to
be hooked, to use the glibc versions of the dirent functions.

Signed-off-by: Thomas Perl <thomas.perl@jolla.com>