Standardize on https://github.com and non-variabilized PKG_SOURCE_URL

Standardize on https://github.com and non-variabilized PKG_SOURCE_URL

This allows for easier extraction of all the repositories used to build
MFW, for instance with:

  perl -lne 'print $1 if m|(http.*untangle/[a-z_]*)|' **/*(.) |  sort -u