Follow-up to 6d972c8b1c which missed updating this directory name.
Also no longer call it crustls in the docs and bump to rusttls-ffi 0.7.1
Closes#7311
This also pins a specific release in the Travis test so future
API-breaking changins in crustls won't break curl builds.
Add RUSTLS documentation to release tarball.
Enable running tests for rustls, minus FTP tests (require
connect_blocking, which rustls doesn't implement) and 313 (requires CRL
handling).
Closes#6763