Ignore missing frame source in thud_abort_frame

Print empty string for missing BASH_SOURCE element in thud_abort_frame.
The last BASH_SOURCE element might be missing sometimes for currently
unclear reason.
Quote strict attr array elements in thud_strict.sh
Add thud_kill_branch

Add thud_kill_branch and requisite thud_get_pid and thud_get_ppid.
Make thud_abort_frame kill process branch

Make thud_abort_frame kill not just $THUD_ABORT_PID or $$, but the whole
process branch between the current shell and $THUD_ABORT_PID. If
$THUD_ABORT_PID is not set, empty, or the process is not found, kill
just the current shell.