Revert r330897:

This was intended to be a non-functional change. It wasn't. The commit
message was thus wrong. In addition it broke arm, and merged crypto
related code.

Revert with prejudice.

This revert skips files touched in r316370 since that commit was since
MFCed. This revert also skips files that require $FreeBSD$ property
changes.

Thank you to those who helped me get out of this mess including but not
limited to gonzo, kevans, rgrimes.

Requested by: gjb (re)

Partial merge of the SPDX changes

These changes are incomplete but are making it difficult
to determine what other changes can/should be merged.

No objections from: pfg

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Fix some edge cases with rewinddir():
- In the unionfs case, opendir() and fdopendir() read the directory's full
contents and cache it. This cache is not refreshed when rewinddir() is
called, so rewinddir() will not notice updates to a directory. Fix this
by splitting the code to fetch a directory's contents out of
__opendir_common() into a new _filldir() function and call this from
rewinddir() when operating on a unionfs directory.
- If rewinddir() is called on a directory opened with fdopendir() before
any directory entries are fetched, rewinddir() will not adjust the seek
location of the backing file descriptor. If the file descriptor passed
to fdopendir() had a non-zero offset, the rewinddir() will not rewind to
the beginning. Fix this by always seeking back to 0 in rewinddir().
This means the dd_rewind hack can also be removed.

While here, add missing locking to rewinddir().

CR: https://phabric.freebsd.org/D312
Reviewed by: jilles
MFC after: 1 week

Hide DIR definition by making it an opaque struct typedef.

Introduce dirfd() libc exported symbol replacing macro with same name,
preserve _dirfd() macro for internal use.

Replace dirp->dd_fd with dirfd() call. Avoid using dirfd as variable
name to prevent shadowing global symbol.

Sponsored by: Google Summer Of Code 2011