Bug 8018: (followup) make the default more explicit

Bug 8018: (followup) make the default more explicit

This patch:
- Makes the new subfield tab show maxlength=9999 as default (instead of
  empty-then-zero).
- Updates the help to make exlpicit that 0 or empty defaults to 9999.
- Assumes all the subfields created with maxlength=0 inadvertedly are
  meant to mean "no limit" and hence update the database to reflect
  that.

To test (this patch and Pablo's):
- Edit a MARC framework, edit some field's subfields.
- Use the 'New' tab to create a new subfield (choose an unused letter).
- See in "More constrains" that the "Max length" field is empty. Leave
  it as-is.
- Save the changes (the new subfield).
- Edit the field again, verify that "Max length" is 0.
- Try tu use the framework and the the field/subfield just created
  > FAIL
- Apply the patches, upgrade
- Try to use the framework/field/subfield > SUCCESS (0 was converted to
  9999)
- Repeat from the begining, "Max length" should show 9999 on the new
  subfield tab.
- Leave it empty, it is saved as 9999.

Edit: small typo

Sponsored-by: Universidad Nacionald de Cordoba
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Passes QA script and tests in t and xt.
Tested:
- deleting an existing subfield
- adding a new subfield with new default 9999
- editing the new subfield, changing value to 8888
- deleting new subfield
- adding new subfield, using 8888 as length

Signed-off-by: Galen Charlton <gmc@esilibrary.com>