MFC r244236,r244408,r244915,r247054,r251512:

MFC r244236,r244408,r244915,r247054,r251512:

  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 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.

git-svn-id: svn://svn.freebsd.org/base/stable/9@252827 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f