Fix Linux, Darwin, and other BSD (untested) to allow proper
handling of xattrs with symlinks. On Linux we cannot use the f*
syscalls for symlinks because symlinks cannot be opened.
File flags must be handled differently on darwin and other BSD due
to the lack of the LCHFLAGS syscall on darwin, and the fact that it
is emulated in libc. However, we do have O_SYMLINK on darwin.
