Charter v69: a Ctrl-C stops the checks one after another, and a check that uses the terminal fails

• A Ctrl-C stops the checks run one after another (#494, found by DEV on PR #493). With the
  checks one after another, the default, a SIGINT did not stop the pre-push checks when the check
  running ended normally on it, as a check that handles SIGINT and exits does: the run went on to
  the next check, and to every one after it. A SIGINT now ends the run with exit 130, a SIGHUP with
  129 and a SIGTERM with 143, in both modes, and the run stops each check with what it started.
• A check that uses the terminal fails, saying so, instead of holding the push (#494, found by
  QA in its review of PR #497). Each check now runs apart from the terminal, one after another too,
  as side by side since v67. A check that reads the terminal, or writes to it once stty tostop is
  set, as git's pager can, is stopped by it: the run now fails that check within about a second,
  saying stopped: it read the terminal, and goes on. Side by side, such a check held the run for
  good. The README's FAQ states the rule for both modes.
• This repository's own tests hold on a loaded machine (#485, #487, #490, #491, #498). Several
  failed pushes here when the Mac was loaded, or could hold the checks for good; each time limit in
  them now only guards against a hang. They do not ship, so nothing changes for a project.
• Upgrading from v68: nothing to do by hand, unless one of the project's checks uses the
  terminal, which now fails it. No session needs a restart: no rule of the charter changes.