Use _ prefix instead of _real suffix on variables

On the whole, the style in manifests for variables that have been
modified in a local scope and saved to a new variable name have shifted
from using `$foo` -> `$foo_real` to using `$foo` -> `$_foo` due to the
shortness of the form and semantic implications that `$_*` is not
neccessarily human-friendly.
Merge pull request #247 from hunner/local_scope_vars

Use _ prefix instead of _real suffix on variables