ha_innobase::check_if_supported_inplace_alter(): Do not allow
adding a FTS_DOC_ID column if a hidden FTS_DOC_ID column exists
due to FULLTEXT INDEX. (This always required ALGORITHM=COPY.)

ha_innobase_inplace_ctx::first_alter_pos: The first column whose position
is affected by instant ADD, DROP, or changing the order of columns.

innobase_instant_try(): Fill in a zero default value for the hidden column
FTS_DOC_ID. Before this, it was not possible to instantly add columns if
a hidden FTS_DOC_ID column existed (at the end of the InnoDB table
definition).