Replace Linuxulator implementation of readdir(), getdents() and
getdents64() with wrapper over kern_getdirentries().

The patch was originally written by emaste@ and then adapted by trasz@
and me.

Note:

1. I divided linux_getdents() and linux_readdir() as in case when the

getdents() called with count = 1 (readdir() case) it can overwrite
user stack (by writing to user buffer pointer more than 1 byte).

2. Linux returns EINVAL in case when user supplied buffer is not enough

to contain fetched dirent.

3. Linux returns ENOTDIR in case when fd points to not a directory.

Reviewed by: trasz@
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D2210