logpersist: incorporate size and buffer tunables

- Add hidden -n and -b flags that propagate to the underlying
  logcat command. Publish aliases in help as --size and --buffer.
- Add --clear for start command so it clears before starting
- shell script bloating up to deal with option validation and
  provide ERROR: and WARNING: advice.

ToDo:

- Deprecate shell script and replace with Eng and Userdebug
  only Developer Options, before it becomes too powerful
  or bloated to force us to rewrite an expendable like this
  into C++.

Bug: 28120456
Change-Id: I0dff8acaa1e5c929454760cfa2801924956bc25d