Warn when RUST_LOG contains a name that isn't a valid path

[jdm]

I can't imagine any reason you would actually want a name that isn't a valid logging identifier.

[catamorphism]

Should be fixed as part of implementing #2033.

[bblum]

That bug is now closed, but this is still an issue.

The place to implement this in the old runtime is in update_log_settings. Not sure how the new runtime will deal with it.

[emberian]

[16:02:15]~/hacking/rust/src/rt> RUST_LOG=foo rustc
warning: got 1 RUST_LOG specs but only matched 0 of them.
You may have mistyped a RUST_LOG spec.
Use RUST_LOG=::help to see the list of crates and modules.

Closing.

[bblum]

Nice!