first step in simplifying the always-update case:

first step in simplifying the always-update case:
tweak @option always-update to allow parsing
@option always-update <hash_value_for_the_whole_plist>

once this is safely in snapshots, we can generate it
directly within PkgCreate.pm, compare it directly in
Signature.pm and get rid of the whole special case
of having to keep the whole plist around