1.0.37.54: more robust runtime executable path detection

1.0.37.54: more robust runtime executable path detection

 Patch by Josh Elsasser.

 * os_get_runtime_executable_path() extended with an argument to
   indicate if the returned path should be externally usable (which
   precludes paths such as /proc/curproc/file).

 * If os_get_runtime_executable_path() fails, use an argv[0] derived
   path instead.

 * Check build_id when saving executable cores to make sure we got
   the right runtime -- since using the argv[0] method can otherwise
   go wrong once in a blue moon.

 Fixes Launchpad #375549.