Re: Reserve prefixes for loaded libraries proposal

Committed.

I made two notable changes:  I renamed the function, since it looked
like EmitWarningsOnPlaceholders() but was doing something not analogous. 

  Also, you had in your function

     strncmp(varName, var->name, varLen)

probably copied from EmitWarningsOnPlaceholders(), but unlike there, we
want to compare the whole string here, and this would potentially do
something wrong if there were a GUC setting that was a substring of the
name of another one.