whitespace cleanup
rustup: make rustup executable
rustup: probe for the existance of tar
rustup: add a RUST_ prefix to the rust-specific variables
rustup: rename TMP_DIR to CFG_TMP_DIR
rustup: factor out the install flags into a CFG_INSTALL_FLAGS variable
rustup: factor out downloading and extracting the snapshot tarballs
rustup: factor out installing packages into a function
rustup: rewrite to protect against truncation

This closes #19168. It's possible that if the downloading of `rustup.sh`
is interrupted, bad things could happen, such as running a naked
"rm -rf /" instead of "rm -rf /path/to/tmpdir". This wraps rustup.sh's
functionality in a function that gets called at the last time that should
protect us from these truncation errors.
rustup: extract the tarballs as part of installation
rustup: simplify downloading packages
auto merge of #19170 : erickt/rust/rustup, r=erickt

This closes #19168.

Please be careful reviewing this since this gets used all over the place. I've tested all the options and everything appears to be working though.