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

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

Cleanup for WARNS 6.

Cleanup for WARNS 3.

Correct the mess I made by committing from the wrong tree. Most
notably, this restores some of the contents in thread_db.h as well
as David Xu's copyright notice. This also fixes the includes in
the MD libpthread files which Scott tried to provide a quick fix
for.

Pointy hat: marcel

Try to fix ia64 and alpha compiles. I don't have either equipment fired
up now, but it appears to be the same problem and solution as sparc64.

Add rudimentary support and stubs for libthr and libc_r on alpha, amd64,
i386, ia64 and sparc64. Add stubs for alpha, amd64, ia64 and sparc64 for
libpthread.

Restructure the source files to avoid unnecessary use of subdirectories
that also force us to use non-portable compilation flags to deal with
the uncommon compilation requirements (building archive libraries for
linkage into a shared library).

The libpthread support has been copied from the original local and
cleaned-up to make them WARNS=2 clean.
that also force us to use non-portable compilation flags to deal with
the uncommon compilation requirements (building archive libraries for
linkage into a shared library).

The libpthread support has been copied from the original local and
cleaned-up to make them WARNS=2 clean.

Tested on: amd64, i386, ia64