New -n option for read builtin

Usage: read -n nchars
Reads maximum of nchars characters. If nchars <= 0, there's no limit which is the default.

As written in #1616, it would be nice to have. Please check the modifications carefully before merging. I'm not an expert.
This is removing the argument from `-m`. You need to do `"...m:n:s"` instead,