Remove the refvar keyword

Remove `refvar` from the language, replacing it with the `ref` keyword.

Doing this requires removing the deprecated `ref(var1,var2)` task intent
syntax. Using that form will now be a syntax error rather than a
warning.

While there I cleaned up buildVarDelcs to take in a set of Flags to
apply to the variable, rather than passing them through three different
variables that would often just be set to FLAG_UNKNOWN and ignored.
If there's a .future handy, it might be nice to note that this error message is not very helpful: "feature request: Better error message"