Add 'scl' builder option for software collection name

With a general 'scl' builder option, a software collection name can be
specified irrespective of the releaser used.  RsyncReleaser's own 'scl'
option has been deprecated in favour of using 'builder.scl' in
releasers.conf.

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Is there somewhere good to document (generic) builder options?  I can't really see much now - maybe I should add a section to the top of doc/builders.mkd?