Let missing Priority field mean optional

Simon Josefssonrequested to merge
jas/dl10n:priority-optional-fix into master

This patch is untested so I appreciate review, something like this seems to be needed to avoid warnings:

https://i18n.debian.org/l10n-pkg-status/c/cppi.html

 The following errors were found in cppi 's PO files: 
 source does not specify a priority

Debian Policy 4.7.3 was updated to allow Priority fields to be optional

https://www.debian.org/doc/debian-policy/ch-archive.html#s-priorities

Some improved wording are discussed but effect remains the same

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1124443

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