devel/cargo: Update to 0.15.0, enable on i386

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Summary

1. Update the port to 0.15.0. The new version requires cargo(1) to be available. Therefore, we let the Ports system to extract the bootstrap cargo (it was done by the Cargo build system before) and set a configure flag to point to it. Also, we need to set OPENSSL_DIR as a Make environment variable because the openssl-sys crate's build.rs doesn't know about FreeBSD.
2. Enable devel/cargo on i386. A bootstrap cargo executable is available for FreeBSD/i386 since last October but I missed it. We can now use it to build a port/package for i386.

Test Plan

Tested with Poudriere on HEAD/amd64 and 11.0/i386.

[dumbbell]

Note that enabling Cargo on i386 will help with future versions of Firefox which will require Rust but not resolve it.