Remove interactive help from core selection parser

Remove interactive help from core selection parser

The help is now handled in the loop that processes user input, and it
never gets into the parser itself.  This removes several special cases
from the parser and simplifies it.

Add a few string utilities and tests for them to make it easy to handle
the help commands.

Change-Id: Ida58bccd3b9ced4b7060485cc2f064b73796c996