pg_execute_from_file, patch v10

Hi,

The other infrastructure patch that has been mark ready for commit then
commented further upon by Tom is $subject, even if the function provided
as been renamed to pg_execute_sql_file().

Please find attached the newer version that fixes Tom concerns, removing
the VARIADIC forms of the functions (those placeholders idea).

The git tree already contains a fixed extension code, but the next patch
for that one will have to wait some more (psql refactoring).

Regards,
-- 
Dimitri Fontaine
http://2ndQuadrant.fr     PostgreSQL : Expertise, Formation et Support