Cargo build should leave the file descriptors opened for the build script

[octplane]

Current code does not forward stdin, stderr, stdout to the spawned build script.

This behavior can be desired if your build script is interactive (vaguely related with #1208).

[alexcrichton]

Unfortunately due to the concurrent nature of Cargo's build process I don't think that this is a feasible feature to implement. We may want some form of a request-response system from a build script to Cargo at some point, but for now it wouldn't work super well if 4 build scripts decided to ask for input all at once :(

[octplane]

Sure. I didn't realize Cargo could multithread its build process...