Improve the UI of 'list-timers'

* lisp/emacs-lisp/timer-list.el (list-timers): Display both "Next"
and "Repeat" in units of seconds, for consistency.
(timer-list-mode): Add help-echo to column headers.
1 job for master in 57 minutes and 58 seconds (queued for 2 seconds)
Status Job ID Name Coverage
failed #5340


Name Stage Failure
test-all Test
make[2]: *** [check-doit] Error 1
Makefile:319: recipe for target 'check-doit' failed
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:294: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:959: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1