pjsip_scheduler.c: Add type ONESHOT and enhance cli show command

* Added a ONESHOT type that never reschedules.

* Added "like" capability to "pjsip show scheduled_tasks" so you can do
  the following:

  CLI> pjsip show scheduled_tasks like outreg
  PJSIP Scheduled Tasks:

  Task Name                                     Interval  Times Run ...
  ============================================= ========= ========= ...
  pjsip/outreg/testtrunk-reg-0-00000074            50.000   oneshot ...
  pjsip/outreg/voipms-reg-0-00000073              110.000   oneshot ...

* Fixed incorrect display of "Next Start".

* Compacted the displays of times in the CLI.

* Added two new functions (ast_sip_sched_task_get_times2,
  ast_sip_sched_task_get_times_by_name2) that retrieve the interval,
  next start time, and next run time in addition to the times already
  returned by ast_sip_sched_task_get_times().

Change-Id: Ie718ca9fd30490b8a167bedf6b0b06d619dc52f3