Consolidate ereport boolean visibility flags into a "flags" field

[dwsteele]

Combine these booleans in elog.h into a single int flags field:

    bool        output_to_server;       /* will report to server log? */
    bool        output_to_client;       /* will report to client? */
    bool        show_funcname;  /* true to force funcname inclusion */
    bool        hide_stmt;      /* true to prevent STATEMENT: inclusion */
    bool        hide_ctx;       /* true to prevent CONTEXT: inclusion */
    bool        hide_from_client;   /* true to prevent client output */