fix(client): force AF_INET socket when resolvePreference is IPv4

[HaizakiKu]

Fixes #1599.

Problem

On Linux systems with IPv4-only virtual interfaces like ip.gre GRE tunnels,
the Hysteria client fails with:

FATAL failed to initialize client {"error": "connect error: timeout: no recent network activity"}

The root cause: Go's net package defaults to dual-stack AF_INET6 sockets when
the network type is "udp". Linux GRE tunnels are strictly IPv4 structures and
silently drop outbound UDP packets arriving from an AF_INET6 socket, causing the
QUIC handshake to time out.

strace confirms the issue -- even when the server address is a plain IPv4 address,
the outbound QUIC socket was created as AF_INET6 and bound to :::

socket(AF_INET6, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, IPPROTO_IP) = 3
bind(3, {sa_family=AF_INET6, ... inet_pton(AF_INET6, "::", ...) ...}, 28) = 0

Fix

After fillServerAddr resolves the server address, fillConnFactory now inspects
the resolved IP and selects the appropriate UDP network type before opening the
socket:

• IPv4 server → "udp4" → AF_INET socket
• IPv6 server → "udp6" → AF_INET6 socket

This applies to both the regular UDP path and the port-hopping path.

Changes

• fillConnFactory: pass resolved network type to so.ListenUDPAddrNetwork
  instead of the implicit dual-stack so.ListenUDP
• add udpNetworkFromAddr helper