cleanup: more consistent abbreviated DS digest type mnemonics

BIND supports the non-standard DNSKEY algorithm mnemonic ECDSA256 everywhere ECDSAP256SHA256 is allowed, and allows algorithm numbers interchangeably with mnemonics. This is all done in one place by the dns_secalg_fromtext() function.

DS digest types were less consistent: the rdata parser does not allow abbreviations like SHA1, but the dnssec-* command line tools do; and the command line tools do not alow numeric types though that is the norm in rdata.

The command line tools now use the dns_dsdigest_fromtext() function instead of rolling their own variant, and dns_dsdigest_fromtext() now knows about abbreviated digest type mnemonics.