pgsql: Several fixes for EXPLAIN (FORMAT YAML), plus one for EXPLAIN

Log Message:
-----------
Several fixes for EXPLAIN (FORMAT YAML), plus one for EXPLAIN (FORMAT JSON).

ExplainSeparatePlans() was busted for both JSON and YAML output - the present
code is a holdover from the original version of my machine-readable explain
patch, which didn't have the grouping_stack machinery.  Also, fix an odd
distribution of labor between ExplainBeginGroup() and ExplainYAMLLineStarting()
when marking lists with "- ", with each providing one character.  This broke
the output format for multi-query statements.  Also, fix ExplainDummyGroup()
for the YAML output format.

Along the way, make the YAML format use escape_yaml() in situations where the
JSON format uses escape_json().  Right now, it doesn't matter because all the
values are known not to need escaping, but it seems safer this way.  Finally,
I added some comments to better explain what the YAML output format is doing.

Greg Sabino Mullane reported the issues with multi-query statements.
Analysis and remaining cleanups by me.

Modified Files:
--------------
    pgsql/src/backend/commands:
        explain.c (r1.196 -> r1.197)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/explain.c?r1=1.196&r2=1.197)