Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

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

Tidy the code up a fraction. Re-release with a 2-clause BSD license
with the kind permission of the author/copyright holder.

Thanks to: phk

Fix grammer in comment.

Submitted by: Engin Gunduz <engin@ripe.net>

Modernize my email-address.

Darn. There is (now was) a signed/unsigned issue that resulted in a
very long loop.

Reported by: nnd@mail.nsk.ru (Nickolay Dudorov)

No functional change, but big code cleanup. WARNS, lint(1) and style(9).

Implement __FBSDID()

Removed duplicate VCS ID tags, as per style(9).

Merge into a single US-exportable libcrypt, which only provides
one-way hash functions for authentication purposes. There is no more
"set the libcrypt->libXXXcrypt" nightmare.
- Undo the libmd.so hack, use -D to hide the md5c.c internals.
- Remove the symlink hacks in release/Makefile
- the algorthm is set by set_crypt_format() as before. If this is
not called, it tries to heuristically figure out the hash format, and
if all else fails, it uses the optional auth.conf entry to chose the
overall default hash.
- Since source has non-hidden crypto in it there may be some issues with
having the source it in some countries, so preserve the "secure/*"
division. You can still build a des-free libcrypt library if you want
to badly enough. This should not be a problem in the US or exporting
from the US as freebsd.org had notified BXA some time ago. That makes
this stuff re-exportable by anyone.
- For consistancy, the default in absence of any other clues is md5. This
is to try and minimize POLA across buildworld where folk may suddenly
be activating des-crypt()-hash support. Since the des hash may not
always be present, it seemed sensible to make the stronger md5 algorithm
the default.
All things being equal, no functionality is lost.

Reviewed-by: jkh

(flame-proof suit on)

Make a dlopen failure consistant with dlsym(). "Shouldn't happen."

Remove -lmd. Use dlopen() and dlsym() instead for calls to the MD5* and
SHA* routines so that callers of libcrypt are not exposed to the internal
implementation.

Big code cleanup. (Inspired by Brandon Gillespie). Also move as
much as possible away from secure/ to make extending easier.