lib/3.6/files.cf: add set_line_based with acceptance tests and deprecate many others

Deprecates `set_config_values`, `set_config_values_matching`, `set_variable_values`, `set_quoted_values`, `maintain_key_values` for a generic reimplementation.

I would prefer to refactor instead of reimplementing, but `edit_line` bundles can't call each other.

@ed45626 @nickanderson @zzamboni @phnakarin please take a look.  The acceptance tests verify that the new `set_line_based` bundle has the same behavior as you'd expect from `set_config_values`, slightly improving the others as previously discussed in #53.