rt: Add a way to configure the runtime environment from the command line

[brson]

We have several environment variables defined in rust_env.cpp for controlling the runtime. It would be nice if this could be done from the command line as well.

See thoughtpolice's comments in #1489.

[graydon]

The +RTS -RTS thing mentioned over in #1489 is ok by me.

[bblum]

If you want to pass +RTS as an actual argument, you can always write ./foo +RTS -RTS +RTS. Like the -- option in many unix utilities.

[bblum]

re-nominating for feature-complete, not backwards-compatible

[pnkfelix]

change in milestone accepted.

[bblum]

beep... beep... beep... triage detector reports no anomalies

[catamorphism]

Just a bug

[brson]

Since the runtime rewrite we no longer have all the runtime configuration stored in the same place, so if we wanted to do this we'd first need to go figure out what all our environment variables are again.

For now there's no demand for this feature.

[thestinger]

I think this is too out-of-date to keep open. A new issue in the same spirit could be reported for the new runtime, but this one lacks context and specifics.