Backport multiple signature support to 4.20

[ffesti]

Multiple signature support will allow using both current and future (post quantum) signatures in a package. As 4.20 will be round for quite a while we want the multiple signature support there, too.

[pmatilai]

AC:

• rpmsign supports --addsign/--resign/--delsign with rpm v6 signatures
• rpmsign can properly handle v4 and v6 package signing:
  • signing defaults vs --rpmv4 and --rpmv6 requests
  • signature space reservation tag differs between v4 and v6 packages
• rpm can verify rpm v4 and v6 signatures on v4 and v6 packages