pgsql: Assorted minor refactoring in EXPLAIN.

Log Message:
-----------
Assorted minor refactoring in EXPLAIN.

This is believed to not change the output at all, with one known exception:
"Subquery Scan foo" becomes "Subquery Scan on foo". (We can fix that if
anyone complains, but it would be a wart, because the old code was clearly
inconsistent.) The main intention is to remove duplicate coding and
provide a cleaner base for subsequent EXPLAIN patching.

Robert Haas

Modified Files:
--------------
pgsql/src/backend/commands:
explain.c (r1.186 -> r1.187)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/explain.c?r1=1.186&r2=1.187)
pgsql/src/backend/lib:
stringinfo.c (r1.50 -> r1.51)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/lib/stringinfo.c?r1=1.50&r2=1.51)
pgsql/src/backend/utils/adt:
ruleutils.c (r1.303 -> r1.304)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/ruleutils.c?r1=1.303&r2=1.304)
pgsql/src/include/lib:
stringinfo.h (r1.36 -> r1.37)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/lib/stringinfo.h?r1=1.36&r2=1.37)