sockets: provide sousrsend() that does socket specific error handling

Sockets have special handling for EPIPE on a write, that was spread out
into several places. Treating transient errors is also special - if
protocol is atomic, than we should ignore any changes to uio_resid, a
transient error means the write had completely failed (see d2b3a0ed31e).

• Provide sousrsend() that expects a valid uio, and leave sosend() for kernel consumers only. Do all special error handling right here.
• In dofilewrite() don't do special handling of error for DTYPE_SOCKET.
• For send(2), write(2) and aio_write(2) call into sousrsend() and remove error handling for kern_sendit(), soo_write() and soaio_process_job().

PR: 265087
Reported by: rz-rpi03 at h-ka.de
Reviewed by: markj
Differential revision: https://reviews.freebsd.org/D35863