Fix decoding of getsockname, getpeername, accept, and accept4 syscalls

Fix decoding of getsockname, getpeername, accept, and accept4 syscalls

* net.c (fetch_socklen): New function.
(decode_sockname): Use it.  On entering syscall, decode early
and return RVAL_DECODED when possible.  On error, print the address
length passed to the kernel.  Do not print more bytes of the socket
address than actually returned by the kernel.