Make prompts and i-searches the same word length

`(reverse-i-search)` and `(i-search)` are different length prompts and so when switching search directions, the entire line shifts position.  I find it disorienting.  Fortunately, `reverse` and `forward` both have seven characters.  This very very minimal patch renames (i-search) to (forward-i-search).  It isn't how readline behaves, but I think it is better.

Similarly, `"help> "` becomes `" help> "` to keep the prompt start at the same location.
Merge pull request #6893 from mbauman/stable-repl-prompt

Make prompts and i-searches the same word length