merger: attributes with multiple dictionaries are not kept

[jayconrod]

mergeExpr breaks an expression into a list and a dictionary (either of which may be empty) with exprListAndDict. This worked well before full multi-platform support was added, but now, generated expressions may have multiple select expressions, so this no longer works.

exprListAndDict will return an error when it encounters an expression it does not understand. When mergeRule gets this error, it will return the generated expression and discard the old expression, regardless of # keep comments.

Two things should be fixed:

• exprListAndDict should be updated to handle expressions with generic, OS-specific, arch-specific, and platform-specific strings (it will need to inspect keys in select statements to do this). Each collection should be merged independently.
• When exprListAndDict returns an error, mergeExpr should return the old expression if there are any # keep comments. Otherwise, it should return the generated expression. It should probably not return an error in any case.