Reduce racket-memory-limit default to 2048 MB.

I think this is better because `custodian-limit-memory` can allow
significant "overshoot". e.g. With the limit at 4 GB I've seen actual
memory usage climb close to 8 GB before the custodian is shut down.

An awful lot of laptops still have only 4 GB total. I don't know, maybe
the default should be even lower. This is a seat of the pants guess.
Fix problem reading quoted path.