pgsql: Restore dblink_current_query() to being a C-language function, so

Log Message:
-----------
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.

Modified Files:
--------------
pgsql/contrib/dblink:
dblink.c (r1.80 -> r1.81)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/dblink.c?r1=1.80&r2=1.81)
dblink.h (r1.21 -> r1.22)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/dblink.h?r1=1.21&r2=1.22)
dblink.sql.in (r1.17 -> r1.18)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/dblink.sql.in?r1=1.17&r2=1.18)