libproc: Improve .gnu_debuglink support.

libproc: Improve .gnu_debuglink support.

As of r278658 libproc looks for debug files under /usr/lib/debug and will
use them if available. This change fleshes out that support a bit further:
- Check for a .gnu_debuglink section and use the file name specified
  there if one is present.
- Validate external debug files with the CRC in the .gnu_debuglink
  section so as to avoid using stale or corrupt debug files.
- Search for debug files in the directory containing the referencing
  object or in the .debug subdirectory, as GDB does.