Quiesce warnings by updating headerfile includes
sh: Allow trapping SIGINT/SIGQUIT after ignore because of '&'.

If job control is not enabled, background jobs started with  ... &  ignore
SIGINT and SIGQUIT so that they are not affected by such signals that are
intended for the foreground job. However, this should not prevent
reassigning a different action for these signals (as if the shell invocation
inherited these signal actions from its parent).

Austin group issue #751

Example:
  { trap - INT; exec sleep 10; } & wait
A Ctrl+C should terminate the sleep command.
A few minor mdoc improvements.
mdoc: document title should be all caps.