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

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

Add a new knob WITH_DEBUG_FILES to control the building of standalone
debug files for userland programs and libraries. The "-g" debug flag
is automatically applied when WITH_DEBUG_FILES is set.

The debug files are now named ${prog}.debug and ${shlib}.debug for
consistency with other systems and documentation. In addition they are
installed under /usr/lib/debug, to simplify the process of installing
them if needed after a crash. Users of bsd.{prog,lib}.mk outside of the
base system place the standalone debug files in a .debug subdirectory.
GDB automatically searches both of these directories for standalone
debug files.

Thanks to everyone who contributed changes, review, and testing during
development.