Add authentication to update-smart-drivedb (ticket #751):

Add signature files drivedb.h.raw.asc to each maintained branch.


git-svn-id: https://svn.code.sf.net/p/smartmontools/code/branches/RELEASE_6_0_DRIVEDB@4359 4ea69e1a-61f1-4043-bf83-b5c94c648137