Auto merge of #2370 - alexcrichton:windows-jobs, r=brson

Auto merge of #2370 - alexcrichton:windows-jobs, r=brson

Currently it's somewhat surprising if you're using cargo and it's then ctrl-c'd.
The child processes that Cargo spawned are likely to still be running around in
the background as they're not killed as well, and this could cause output spew
or future build failures.

This situation is handled by default on Unix because ctrl-c will end up sending
a signal to the entire *process group*, which kills everything, but on Windows
we're not as lucky (just Cargo itself is killed). By using job objects on
Windows we can ensure that the entire tree dies instead of just the top Cargo
process.

cc #2343