ha_thd() whenever possible.

EQ_CURRENT_THD(thd): New predicate, for use in assertions.

innobase_drop_database(): Tolerate current_thd == NULL, so that the
Windows plugin will work.  In the Windows plugin, it will be
impossible to skip foreign key checks in this function.  However,
DROP DATABASE will drop each table (that MySQL knows about) individually
before calling this function.  Thus, the foreign key checks can be disabled
also in the Windows plugin, unless some .frm files are missing.