Standardize SFU MOD tags in core modifications

Wrap modifications with comments that start with "SFU MOD CANVAS-XYZ"
and end with "END SFU MOD". Use "SFU MOD - CANVAS-XYZ" for one-liners.
Merge pull request #118 from va7map/standardize-sfu_mod

Standardize SFU MOD tags in core modifications