rustpkg ignoring `#[link(name="foo")]`

[emberian]

There seems to be a bug where rustpkg ignores the #[link(name="foo")], both when building executables (for example, on rustdoc_ng it emits a "main" executable rather than "rustdoc_ng"), and installing (for rust-zmq, it tries to find librust_zmq-etc.so, even though it's built as libzmq)

[emberian]

With rustdoc_ng it seems to work fine now, but for rust-zmq it's still a problem

[catamorphism]

I suspect rustpkg is ignoring #[link(vers=3.1415927)] as well, and that really should work.

This also will need documentation once it's finished.

[catamorphism]

@graydon thinks #[link(...)] should be removed altogether, so as to make it possible to refer to packages in extern mod directives. That change would require consensus, I guess.

[catamorphism]

Closing in anticipation that #8523 will be accepted.