accept4() support

[MikeHawkesCapventis]

Knowing that the go compiler crashes, I attempted to run a small web-server by cross-compiling Go code. The code runs, but falls over when attempting to set up the network socket, returning "TCP accept: not implemented" - dmesg shows "FIXME socketcall 18".

One of the friendly folk on Discord pointed me towards fs/sock.c and kernel/calls.c and it looks like the code doesn't support accept4 in the socket library (calls.c passes to a stub handler). It's a real shame as this prevents any Go web-application from being compiled for / ported to iSH.