pgsql: Export ExplainBeginOutput() and ExplainEndOutput() for

Log Message:
-----------
Export ExplainBeginOutput() and ExplainEndOutput() for auto_explain.

Without these functions, anyone outside of explain.c can't actually use
ExplainPrintPlan, because the ExplainState won't be initialized properly.
The user-visible result of this was a crash when using auto_explain with
the JSON output format.

Report by Euler Taveira de Oliveira.  Analysis by Tom Lane.  Patch by me.

Modified Files:
--------------
    pgsql/contrib/auto_explain:
        auto_explain.c (r1.8 -> r1.9)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/auto_explain/auto_explain.c?r1=1.8&r2=1.9)
    pgsql/src/backend/commands:
        explain.c (r1.194 -> r1.195)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/explain.c?r1=1.194&r2=1.195)
    pgsql/src/include/commands:
        explain.h (r1.42 -> r1.43)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/explain.h?r1=1.42&r2=1.43)