Restore dblink_current_query() to being a C-language function, so as to not

Restore dblink_current_query() to being a C-language function, so as to not
create an ABI break between 8.3 and 8.4.  It is still just a wrapper around
the built-in current_query() function, but at a different implementation
level.  Per my proposal.

Note: this change doesn't break 8.4beta installations, since their
SQL-language definition of the function still works fine.