install: honour CARGO_TARGET_DIR and support prefixes properly
On top of !9 (merged) to avoid unnecessary rebases
Adds --verbose --dry-run --help
and adjusts the udev rule to honor the prefix if given. Also now honours CARGO_TARGET_DIR
if set (but doesn't handle the case where that is set via the config.
Also, I'm a bit unclear about CARGO_TARGET_DIR
vs CARGO_BUILD_TARGET_DIR
so it doesn't handle the latter atm.