[libc] Add struct ipv6_mreq and IPv6 socket options (#206448)

This patch adds struct ipv6_mreq and defines various IPv6 socket option
macros in <netinet/in.h>. I've also moved the existing option
definitions to the yaml file.

Assisted by Gemini.

GitOrigin-RevId: 831ad2ee6af862965c21d7b785b0b00ed7bb7d9d