Report back traces after finding core files

When core files are found, run this on them:    core_bt -t
The resulting gdb command for each file is:    thread apply all bt

In second patch set, fully qualify core_bt, i.e., invoke
/usr/local/bin/core_bt

Change-Id: I0b1f19adec4c8e23e2fd03c9127c315aa99949bd
Merge "Report back traces after finding core files"