This commit changes the shebang in all shell scripts from absolute
paths (such as `/bin/bash` or `/usr/bin/python3`) to the `/usr/bin/env`
binary with the accompanying interpreter as it's argument.
The reason for this are as follows:
- NixOS cannot work with absolute paths
- BSD systems install their packages in /usr/local/bin
This commit puts the native-tls crate behind a feature. The feature
is off-by-default in the tor-rtcompat crate, but can be enabled
either from arti or arti-client.
There is an included script that I used to test that tor-rtcompat
could build and run its tests with all subsets of its features.
Closes#300