MODE ( INNODB ONLY )

Unlike, MyISAM, InnoDB does not support Boolean full text search (FTS) on
non-indexed columns.  Before this fix, code allowed non-indexed columns
in MATCH expression for BOOLEAN mode, but not for NATURAL LANGUAGE mode.
This patch fixes this issue by restricting the exception for BOOLEAN mode to 
only apply to MyISAM and not to InnoDB.

This is patch is a bit ad-hoc in the way it determines whether to allow
non-indexed columns or not.  It exploits the fact that InnoDB supports
an extended FTS API, while MyISAM does not.  In other words, non-indexed
columns are only allowed if the storage engine does NOT support the 
extended FTS API.

A side effect of this patch is that when reporting an error due to 
non-indexed columns in MATCH expression, we will no longer distinguish
between tables without any fulltext indexes and tables with no applicable
fulltext indexes.  In the first case, the error used to be
ER_TABLE_HAS_NO_FT.  Now, one will get ER_FT_MATCHING_KEY_NOT_FOUND in 
both cases.  This matches the existing behavior of NATURAL LANGUAGE mode 
for both InnoDB and MyISAM.