Introduce a separate function to copy from msghdr32 to msghdr

Introduce a separate function to copy from msghdr32 to msghdr

This patch is an initial step for supporting "-e write=set" and
"-e read=set" option for sendmmsg and recvmmsg system calls.

Coverting a data of msghdr32 to msghdr is needed both for
{send,recv}msg and {send,recv}mmsg to decode parameters.
To share the copying code in both decoders, a separate
function named copy_from_msghdr32 is introduced.

* net.c [SUPPORTED_PERSONALITIES > 1 && SIZEOF_LONG > 4]
(copy_from_msghdr32): New function.
(extractmsghdr) [SUPPORTED_PERSONALITIES > 1 && SIZEOF_LONG > 4]: Use it.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>