tests: cleanup trapping of signal names

tests: cleanup trapping of signal names

A side effect of this cleanup is we no longer
depend on our own kill command being built.

* init.cfg (require_trap_signame_): A new function to verify
that the shell supports specifying traps by signal name.
(require_kill_group_): A new function to ensure the shell
supports sending a signal to a group.
* tests/du/move-dir-while-traversing.sh: Ensure trap supports
signal names.
* tests/misc/stty-invalid.sh: Likewise.
* tests/misc/stty-pairs.sh: Likewise.
* tests/misc/stty-row-col.sh: Likewise.
* tests/misc/stty.sh: Likewise.
* tests/misc/sort-compress.sh: Likewise.  Also simplify trap call.
* tests/install/trap.sh: Likewise.
* tests/misc/timeout.sh: Likewise.
* tests/dd/stats.sh: Likewise.  Also use default kill command.
* tests/misc/timeout-group.sh: Likewise.