Add getdirentries() and stat() for Mac OS X.

Add getdirentries() and stat() for Mac OS X.

Also add fstat() and lstat().  I don't #define the constants for
st_mode or d_type, but I provide enough to get the block size of a
file and to list the names in a directory.  Some fields of struct stat
get truncated, see XXX in plat/osx/include/sys/stat.h.

In struct dirent, the inode field might be d_ino or d_fileno.  I
picked d_ino because Apple's sys/dirent.h uses d_ino (but Apple's
manual pages use d_fileno).