Bug 35079: Replace --force-extract by --generate-pot={always,auto,never}

Bug 35079: Replace --force-extract by --generate-pot={always,auto,never}

This restores the original behaviour of always building the POT file
in order to not break existing workflows
Option --force-extract is deleted in favor of a new option
--generate-pot that can have 3 values:

* always: always build the POT file. This is the default value
* auto: build the POT file only if it does not exist
* never: never build the POT file

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>