Add basic support for AF_NETLINK socket type

[shoenig]

This PR adds a new "netlink" promise which enables making use of socket with type AF_NETLINK, type parameter SOCK_DGRAM or SOCK_RAW and protocol parameter NETLINK_ROUTE. Some applications make use of netlink sockets for IPC with the kernel to communicate network related information.

There are many more protocols in the netlink family, but NETLINK_ROUTE is the one I need right now; we can add more in the future if the use cases reveal themselves.

My specific immediate use case is in trying to run the envoy proxy under pledge, where without this change it aborts with

[pid 13114] socket(AF_NETLINK, SOCK_RAW|SOCK_CLOEXEC, NETLINK_ROUTE <unfinished ...>
[pid 13116] <... rt_sigprocmask resumed>NULL, 8) = 0
[pid 13116] futex(0x7f87c76c0340, FUTEX_WAIT_PRIVATE, 0, NULL <unfinished ...>
[pid 13114] <... socket resumed>)       = -1 EPERM (Operation not permitted)

${PLEDGE} \
  -p "stdio rpath netlink recvfd inet dns exec" \
  -O 1024 \
  -v "r:./envoy-minimal.yaml" \
  /opt/bin/envoy-x86_64 \
    --disable-hot-restart \
    --concurrency 2 \
    --config-path ./envoy-minimal.yaml

Bear in mind I'm not familiar with

• Programming in C
• Programming eBPF
• Netlink
• OS syscalls

so I apologize if this is out in the weeds 🙂