readline: don't emit "line" events with a trailing '\n' char

Before this commit, readline was inconsistent in whether or not it would emit
"line" events with or without the trailing "\n" included. When "terminal"
mode was true, then there would be no "\n", when it was false, then the "\n"
would be present. However, the trailing "\n" doesn't add much, and most of the
time people just end up stripping it manually.

Part of #4243.