Make rustpkg accept multiple package IDs in a single command

[catamorphism]

rustpkg should be able to accept more than one package ID in a single command. The reason to do this is that multiple parallel rustpkg processes will have unpredictable results, because rustpkg currently doesn't lock the workcache database file in any way. To get around this, we should allow rustpkg to accept multiple package IDs (it will then build them in parallel). We should also document that it's a bad idea to run multiple rustpkg instances in parallel, but I'll make another issue for that.

[thestinger]

rustpkg is gone