logcat: Add -h and --help flags

If -h or --help argument was supplied, logcat would report the help
message but also return an error.  Officially add -h and --help as
recognized flags, report all help with a (zero) success error code.
Adding this, and the associated test, was split off as a stepping
stone to resolving an issue with logcat -L or --last flag operations.

Test: gTest logcat-unit-tests --gtest_filter=*.help
Bug: 28788401
Bug: 30041146
Bug: 30612424
Bug: 35326290
Change-Id: I948e7fa4e92bd23f52717758ffd96bbd068c53d4