cleanup: more consistent abbreviated DS digest type mnemonics

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.