REORG: connection: move the socket iocb (conn_fd_handler) to sock.c

REORG: connection: move the socket iocb (conn_fd_handler) to sock.c

conn_fd_handler() is 100% specific to socket code. It's about time
it moves to sock.c which manipulates socket FDs. With it comes
conn_fd_check() which tests for the socket's readiness. The ugly
connection status check at the end of the iocb was moved to an inlined
function in connection.h so that if we need it for other socket layers
it's not too hard to reuse.

The code was really only moved and not changed at all.