Remove trailing whitespace
Update document dates
Move archive extraction to a separate function.

According to the requirement of compatibility with all repository
styles, move archive extraction to a separate function in update
code. So extracting and signature checks can be performed for all
repositories: both sqlite and yaml ones.

Reorganize constants slightly while I'm here.
Organize full repo update as separate function.

To handle both types of repos it is necessary to have full update
as a separate procedure.
Fix const variables.

Use constant pointers to access constant array members of upgrade
schema.
Make private consts static.

Const variables declared in headers should be static as well.