MFC: fstest regression fixes - POSIX error codes.

MFC: fstest regression fixes - POSIX error codes.

Partial MFC of d7c75c7a20590cab729c3d653aaa91a4960d6165:

Deal with most of the issues found by FreeBSD's fstest regression test:
    * Limit path components to 255 characters.  Return ENAMETOOLONG if
      the limit is exceeded.

    * Return EEXIST, EINVAL, and ENOTEMPTY as appropriate when the user
      attempts to create, delete, or rename "." or "..", instead of EINVAL.

    * Return EACCES if an attempt is made to open a file O_TRUNC without
      O_RDWR or O_WRONLY, instead of silently dropping the O_TRUNC.

    * Implement O_NOFOLLOW semantics generally instead of just with O_EXCL.

Not MFC'd:
    * Return EISDIR if an attempt is made to open a directory for writing,
      instead of EINVAL.

This is required so that /usr/bin/tar can extract to an existing
directory:

	tar cf - . | (cd otherdir && tar xof -)

failed before (breaking nrelease).