Checking that xargo isn't already installed before trying to install …

Created by: HarryU

…it - should prevent the bootstrap script erroring out in an ugly way.

Problem: [describe the problem you try to solve with this PR.] The bootstrap script finished with an ugly error when trying to run cargo install xargo if xargo was already installed. This didn't seem to actually break anything, but meant the script didn't run the statusCheck function, remove the bootstrap.sh file or tell the user the commands to run in order to build and run Redox. Solution: [describe carefully what you change by this PR.] I have added a check which uses cargo install --list to examine the install crates and check for xargo, only installing it if it isn't present. I hope this will work on any system, but I can only try it on Ubuntu 14.04 and 16.04 at the moment.

Changes introduced by this pull request:

Drawbacks: [if any, describe the drawbacks of this pull request.]

TODOs: [what is not done yet.]

Fixes: [what issues this fixes.]

State: [the state of this PR, e.g. WIP, ready, etc.] I think it is ready to merge. Blocking/related: [issues or PRs blocking or being related to this issue.]

Other: [optional: for other relevant information that should be known or cannot be described in the other fields.] This is my first pull request to an OS project - please let me know if I've done anything wrong!