MFC r310931,r310942,r310988:

r310931:

Use strdup in snmp_parse_server(..) when possible instead of malloc+strcpy

This simplifies the code and mutes a Coverity warning about sc->cport being
improperly allocated

CID: 1018247

r310942:

Unbreak the build by passing the string to strdup, not its length

Pointyhat to: ngie

r310988:

snmp_discover_engine: fix up req/resp (PDU object) handling a bit

• Call snmp_pdu_free on req and resp when done with the objects
• Call snmp_pdu_free on req before calling snmp_pdu_create on it again