Improved tab completion for FDW DDL

Hi,

Here is a patch which adds the below missing tab completions for the FDW
DDL commands. I noticed these were missing while playing around with
FDWs a while ago.

"ALTER SERVER <name>" with "RENAME TO"
"CREATE SERVER <name> FOREIGN DATA WRAPPER" with fdw name
"CREATE|ALTER USER MAPPING FOR <name> SERVER <name>" with "OPTIONS ("

Another completion which is currently missing but I am not sure if we
should add or not is completing "ALTER|CREATE|DROP USER" with "MAPPING
FOR", but since it might interfere with completing to username for
"ALTER|DROP USER" I am not sure we want it. What do you think?

Andreas