SpawnProcess: improve dummy pipe allocation logic

By using allocated file descriptors for keys in fd_pipes, we naturally
avoid interference with callers such as FileDigester and MergeProcess.
See the _setup_pipes docstring for more benefits of this allocation
approach.
EbuildMetadataPhase: use dynamic pipe fd

Also, remove deprecated code from doebuild (with hardcoded pipe fd).
doebuild: tweak handling of _unused param