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

Log Message:
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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:
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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)