Fix auto-scrolling on long lines

[dbeecham]

If the cursor is on the last line, the output of the terminal window is always auto-scrolled to the last line, so I removed the line-counting logic and jumped to the last line when the buffer is created.

I also added a some various smaller improvements;

1. nvim_open_term automatically converts \n to \r\n, so the regex loop is not needed
2. Rewrote the "header" logic slightly for readability and allows header strings longer than 6 characters
3. Sent the terminal message separately, as it's theoretically allowed to contain nulls, which string operations does not

Thank you.

[michaelb]

LGTM, thanks for contributing.

I will release a new version after merging this and a couple commits from the development branch EDIT: I'm realizing I may not need to release a new version as only the Lua part of the code is changed, and releases are useful to distribute changes of the Rust (compiled) part. I usually can't skip issuing a new release, but for the current changes it's perfectly appropriate