closes #9274
ref #9028

If stdin closed then read_error_exit calls preserve_exit. Handling
SIGHUP during preserve_exit would cause a premature teardown, and
conflicts with e.g. ui_bridge_stop which waits for TUI to teardown.

Vim ignores SIGHUP in its prepare_to_exit and getout_preserve_modified
routines:

    /* Ignore SIGHUP, because a dropped connection causes a read error, which
     * makes Vim exit and then handling SIGHUP causes various reentrance
     * problems. */
    signal(SIGHUP, SIG_IGN);

1. Don't check elapsed time in children_kill_cb(), it's already implied
   by the start-time of the timer itself.
2. Restart timer from children_kill_cb() for PTY jobs, to send SIGKILL
   after SIGTERM. There is an edge case where SIGKILL might follow
   SIGTERM too quickly, if jobstop() is called near the 2-second timer
   window.  But this edge case is not worth code complication.

Implement nvim_command_output with `execute({cmd},"silent")`.

Behavior changes:
- does not provoke any hit-enter prompt
- no longer prepends a newline char
- does not capture some noise (like the "[New File]" message, see the
  change to tabnewentered_spec.lua)

Technically ("bug-for-bug") this a breaking change.  But the previous
behavior of nvim_command_output meant that it probably wasn't used for
anything outside of tests.

Also remove the undocumented `v:command_output` variable which was
a hack introduced only for the purposes of nvim_command_output.

closes #7726