Draft: backtrace dump for all capability armed threads on SIGQUIT (addressing #15774)

(this MR rebases !4786 (closed) onto master so supersedes it)

This is an attempt to move a tiny step forward addressing #15774

• are either individually buildable or squashed
• have commit messages which describe what they do (referring to [Notes][notes] and tickets using #NNNN syntax when appropriate)
• have added source comments describing your change. For larger changes you likely should add a [Note][notes] and cross-reference it from the relevant places.
• add a testcase to the testsuite.
• replace this message with a description motivating your change