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 comment modification is also unnecessary. Functions referred to in comments typically omit the arguments. If you need to distinguish between overloads, you'd use the type name here instead, but there's no overloads of `reader_readline()`.