Refactor network interface handling

• Refactor nas_config.c to remove a lot of duplicated code.
• Remove unused code
• Implement the handling of IPv4v6 and IPv6 PDU sessions. Note that as of now, the UE only requests IPv4 sessions (need to modify the code for the others), as the default configuration for OAI 5GC seems to reject any non-IPv4 PDU session.

Still missing:

• test all modes
• enable IPv6 handling in nas_config()
• remove netlink and --nokrnmod option?
• Finalize TODO commit: can remove unused code in 4G PDCP
• move to separate module? (to be discussed) I already removed a lot of netlink code, so after everything is done, I will create a small module common/utils/if_tun.c
• use nas_config() in 4G NAS decoding
• fix issue regarding rules: see !2848 (comment 128171)