pgsql: Split out MERGE and REPLACE/UPSERT items.

Log Message:
-----------
Split out MERGE and REPLACE/UPSERT items.

< * Add MERGE command that does UPDATE/DELETE, or on failure, INSERT (rules,
< triggers?)
> * Add SQL-standard MERGE command, typically used to merge two tables
>
> This is similar to UPDATE, then for unmatched rows, INSERT.
> Whether concurrent access allows modifications which could cause
> row loss is implementation independent.
>
> * Add REPLACE or UPSERT command that does UPDATE, or on failure, INSERT

Modified Files:
--------------
pgsql/doc:
TODO (r1.1706 -> r1.1707)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/TODO.diff?r1=1.1706&r2=1.1707)
pgsql/doc/src/FAQ:
TODO.html (r1.213 -> r1.214)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/FAQ/TODO.html.diff?r1=1.213&r2=1.214)