Switch default snmpd and snmp auth back to hmac-sha1.

Switch default snmpd and snmp auth back to hmac-sha1.

Practical experience on several machines after updates suggests the snmp
world isn't really ready for hmac-sha2-256, and the HMAC construction doesn't
require collision resistance (which is the weakness of MD5/SHA1; see e.g.
"New proofs for NMAC and HMAC: Security without collision-resistance",
Bellare 2014).

Feedback from martijn@ (who would prefer to keep using the sha2 hmac),
deraadt@, tb@.