addrblock: Allow limiting validation depth of issuer addrblock extensions

addrblock: Allow limiting validation depth of issuer addrblock extensions

RFC3779 requires to validate the addrblocks of issuer certificates strictly,
that is, they must contain the extension and the claimed addrblock, up to
the root CA.

When working with third party root CAs that do not have the extension,
this makes using the plugin impossible. So add a depth setting that limits
the number of issuer certificates to check bottom-up towards the root CA.
A depth value of 0 disables any issuer check, the default value of -1
checks all issuers in the chain, keeping the existing behavior.

Closes strongswan/strongswan#860