Address #229: Improve documentation and error output on MacOS when not running as root

[zbentley]

Addresses/mitigates #229

Two changes in this PR:

• Force an error when running via cargo flamegraph on MacOS without the --root flag.
• Update documentation regarding the reasons for preferring the --root flag over sudo cargo.

Additional things that I could do if needed:

• Add equivalent runtime errors to the non-cargo standalone flamegraph binary. I didn't do that because dtrace will indicate an error anyway without permissions, and sudo flamegraph isn't as much of a problem as sudo cargo.
• Force an error if euid==0 in the cargo flamegraph binary, telling people not to run cargo as root. That seems overly perscriptive as there are probably some folks who intentionally do this.
• Actually dry-run dtrace fully against a dummy program, intercept error output, and provide suggestions based on the presence of known errors. This would potentially be useful (and not just on MacOS) but would be a lot more work.

[zbentley]

@djc you want me to bump shlex to resolve that audit advisory build failure?

I did this as the new version seems harmless, as this crate doesn't seem to be depending on any of shlex's functionality that was deprecated or changed while fixing the security vuln.