Enable readline in phpdbg

[computator]

phpdbg and readline are already both enabled by default, however the --enable-phpdbg-readline flag is not set to enable readline in phpdbg. If this could be enabled it would make using phpdbg much more convenient!

[tianon]

If phpdbg is enabled, it's definitely not on purpose. 😬 🙈

[computator]

Well it's definitely useful, so don't disable it or anything, or at the least leave it enabled in the CLI variants please :)

[tianon]

Apparently we were excited to have it implicitly (looking through old issue comments), so I've codified the explicit stance (and included readline) in #1259. 👍

[computator]

Awesome, thanks!