unetmsg: fix TCP_USER_TIMEOUT socket option level

unetmsg: fix TCP_USER_TIMEOUT socket option level

The ucode socket module exports no SOL_TCP so setopt() was called
with level 0 and silently failed. This left inter-node connections
without the 5 second user timeout. A peer that stopped reading
could then stall a blocking channel write forever, deadlocking
every unetmsgd in the mesh along with their local clients.

Use IPPROTO_TCP and warn when setopt fails.

Signed-off-by: Chad Monroe <chad@monroe.io>
Link: https://github.com/openwrt/openwrt/pull/24595
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>