luci-app-snmpd: add strong SNMPv3 crypto options

luci-app-snmpd: add strong SNMPv3 crypto options

Add SHA-256, SHA-384, SHA-512, SHA-224 authentication and AES-192,
AES-256 encryption options to the SNMPv3 user configuration.

net-snmp has supported these algorithms for a while, but the LuCI
dropdown only offered SHA(-1), MD5, AES(-128) and DES. Modern security
standards (e.g. EN 18031 CCK-1) require at least 112-bit security
strength, which rules out SHA-1 (80-bit) and DES (56-bit).

The default authentication type is changed from SHA to SHA-256.

The init script already passes auth_type and privacy_type directly to
net-snmp's createUser, so no backend changes are needed.

Signed-off-by: Michael Pfeifroth <redacted>