Remove needless comment
Add mroonga_boolean_mode_syntax_flags system and session variable

You can customize syntax `MATCH () AGAINST ("..." IN BOOLEAN MODE)`.

Available flags:

  * DEFAULT: Equals to SYNTAX_QUERY,ALLOW_LEADING_NOT.
  * SYNTAX_QUERY: Uses query grnexpr syntax. If neither SYNTAX_QUERY nor
    SYNTAX_SCRIPT aren't specified, SYNTAX_QUERY is used.
  * SYNTAX_SCRIPT: Uses script grnexpr syntax. If both SYNTAX_QUERY and
    SYNTAX_SCRIPT are specified, SYNTAX_SCRIPT is used.
  * ALLOW_COLUMN: Allows "COLUMN:..." syntax in query grnexpr syntax.
  * ALLOW_UPDATE: Allows updating value byb "COLUMN:=NEW_VALUE" syntax
    in query grnexpr syntax.
  * ALLOW_LEADING_NOT: Allows "-NOT_INCLUDED_KEYWORD ..." syntax
    in query grnexpr syntax.

TODO:

  * Document me.