nsysnet: Some improvements

[GaryOderNichts]

This PR changes the following:

• Adds missing WU sockopt types
• Avoids always raising an assert for unimplemented types
• Properly handle NULL src_addr and addrlen in recvfrom
• Implements getsockopt with SO_TYPE to query the socket type

[Exzap]

Looks good to me, thanks!