Support multiple folders in DBD::File's f_dir attribute

No tests yet and a few locations where things still will go wrong
when the catdir () is involved
Use a new attribute: f_dir_ext

Extended dir search now more explicit. One of the reasons not to use
a ref in f_dir is that the current definition is (very) clear and we
should not break that. What e.g. would we use to write new files?

If a table is opened from folder 2, then deleted and recreated, does
that mean we should re-use folder 2? If f_dir working as before this
problem disappears
Jens really prefers f_dir_search over f_dir_ext
Make deep searches work.

Tested with DBD::CSV, now all we need is sanity checks and agreement on
this new feature
Tests for f_dir_search
stop confusing duplicate defining @tbl ...
Clarify bind_col TYPE attribute is sticky
Spelling fixes.

Signed-off-by: H.Merijn Brand - Tux <h.m.brand@xs4all.nl>
Give Ville the credits for his patch

As I used git-am, I could not include this change in the previous
commit, unless I modified the patch to include this change :)
one more typo
Fix handling "0" different from ""

Handle strings evaluating to a false value correct when simply expecting
data in DBI::SQL::Nano_::Statement part approval
Support multiple folders in DBD::File's f_dir attribute

No tests yet and a few locations where things still will go wrong
when the catdir () is involved
Jens really prefers f_dir_search over f_dir_ext
Merge remote-tracking branch 'origin/f_dir-ref' (and updated Changes)

Conflicts:
	Changes