Add Protocol type and socket_with_protocol() for arbitrary protocol numbers

[batkovic75]

Summary

Add a new Protocol type and socket_with_protocol() function that allow creating sockets with arbitrary protocol numbers not defined in the SockProtocol enum.

This is particularly useful for AF_PACKET sockets where Ethernet protocol numbers (like ETH_P_ARP, ETH_P_8021Q, etc.) need to be specified in network byte order.

New APIs

• Protocol::new(c_int) - create from raw protocol number (all platforms)
• Protocol::ethernet(u16) - create from Ethernet protocol with automatic byte order conversion (Linux/Android)
• Protocol::as_raw() - get the raw value
• socket_with_protocol() - like socket() but accepts Protocol
• impl From<SockProtocol> for Protocol - convert existing protocols

Example

use nix::sys::socket::{socket_with_protocol, AddressFamily, SockType, SockFlag, Protocol};

// Create a raw socket to capture ARP packets
let proto = Protocol::ethernet(libc::ETH_P_ARP as u16);
let sock = socket_with_protocol(AddressFamily::Packet, SockType::Raw, SockFlag::empty(), proto)?;

Design Rationale

This takes an additive approach rather than modifying the existing SockProtocol enum:

• Existing socket() API remains unchanged
• No breaking changes
• Users who need arbitrary protocols can opt-in to the new API

Related PRs took different approaches:

• SockProtocol::ETH_P_* SockProtocol::Htons(u16) and SockProtocol::Integer(i32) #865 proposed adding Htons(u16) and Integer(i32) variants (abandoned)
• Switch to SockProtocolInt #2068 proposed replacing SockProtocol with a wrapper type everywhere (stale)
• sys::socket adding Linux packet filtering on ipv4/ipv6/loopback traffics #2581 (merged) added common Eth* variants - this PR provides an escape hatch for the rest

Closes #854

Test plan

• Added test_protocol_ethernet test that verifies byte order conversion and socket creation
• All existing tests pass
• Clippy passes