Support preferLocalBuild

Derivations with "preferLocalBuild = true" can now be executed on specific machines (typically localhost) by setting the mandary system features field to include "local". For example:

localhost x86_64-linux,i686-linux - 10 100 - local

says that "localhost" can only do builds with "preferLocalBuild = true". The speed factor of 100 will make the machine almost always win over other machines.

Created by  Eelco Dolstra  on June 29, 2015
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