backport: DNSSEC Single-Type Signing Scheme

Original commits:

6319974e  Merge branch 'dnssec-per-algorithm-single-type-signing' into 'master'
13a040d8  review: fix comments
824457af  fix compatibility with old compilers
e872962e  DNSSEC tests: algorithm dependent Single-Type Signing Scheme
3c54c086  DNSSEC: algorithm dependent Single-Type Signing Scheme

7cf00157  Merge branch 'dnssec-single-type-signing' into 'master'
ae26a394  DNSSEC: single-type signing documentation
41b0aab1  tests-extra: consider single-type signing in dnskey_algorithms
17c77f4c  tests-extra: DNSSEC Single-Type Signing Schema
844b53a3  tests-extra: dnssec-verify, ignore KSK flag setting
db4d4616  DNSSEC: allow Single-Type Signing Scheme
16d2231a  DNSSEC: handle KSK and ZSK independenlty, do not sign DNSKEY with ZSK