lsock_init_port: address issues with initializing sockaddr_un object

• Use strlcpy to ensure p->name doesn't overflow sa.sun_path [*].
• Use SUN_LEN(..) instead of spelling out calculation longhand (inspired by comment by jmallett).

Tested with: dgram and stream support with both bsnmpwalk and snmpwalk

MFC after: 1 week
Reported by: Coverity
CID: 1006825