Add functions for dumping iovecs in mmsghdr used in sendmmsg and recvmmsg

Add functions for dumping iovecs in mmsghdr used in sendmmsg and recvmmsg

This patch is similar to what I did in commit
02f9f6b386741a52f58e1b31ad4e7fff60781ef8.
That commit was for sendmsg and recvmsg system calls.
This one is for sendmmsg and recvmmsg system calls.

* defs.h (dumpiov_in_mmsghdr): New declaration.
* net.c (extractmmsghdr): New function derived from printmmsghdr.
(printmmsghdr): Use it.
(dumpiov_in_mmsghdr): New function.
* syscall.c (dumpio) [HAVE_SENDMSG]: Call dumpiov_in_mmsghdr
for recvmmsg and sendmmsg syscalls.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>