Delegate Ctrl+C handling for shims

[charlespierce]

Closes #500

Info

Currently the Ctrl+C SIGINT signal is being sent to both the Volta process and the shimmed child process simultaneously. This means that if a shim waits after receiving the SIGINT signal (as in the repro example in #500), Volta will still exit immediately and not wait for the shim.

Changes

• Used the ctrlc crate to set a handler for Ctrl+C in the shim executable.
• Handler uses a flag to determine if control has been passed to the shim or not.
  • If it has, the handler does nothing and lets the child process control the flow of the application.
  • If it hasn't, then we are still doing Volta work (perhaps downloading a version of Node), so it exits immediately (the default behavior).
• Added a call to set the flag immediately before running the shim process.

Tested

• Confirmed that the reproduction case in the issue is handled correctly, the process waits for Node to exit before returning control to the user.
• Verified that interrupting the automatic download of a tool still correctly interrupts immediately.
• Tested on both Mac & Windows to ensure that the behavior is accurate cross-platform.