Bug 10214 Add header to syspref po files

Bug 10214 Add header to syspref po files

With this patch, header is created when creating a new syspref .po file
for a new language (translate create), and is added if it doesn't
already exist when updating an existing language (translate update).

To test:

(1) Create a new language syspref file:

    ./translate create -p xx-XX

    Check that there is an header

(2) Update an existing syspref file without header:

    ./translate update fr-FR

    Check that fr-FR-pref.po has a header

(3) Update an existing syspref file with header:

    Modify fr-FR-pref.po. Add an email, or whatever.

    ./translate update fr-FR

    Check that fr-FR-pref.po has a header with the manual modification

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Comment: Work as described. No errors.

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
All tests and QA script pass.
Also tested that updated pref files can still be installed correctly.
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>