Add a function for dumping iovec in msghdr used in sendmsg and recvmsg

Add a function for dumping iovec in msghdr used in sendmsg and recvmsg

Here is an example session:

    $ ./strace -e write=all ip link change dev enp0s25 mtu 1501 > /dev/null
    sendmsg(3, {msg_name(12)={sa_family=AF_NETLINK, pid=0, groups=00000000}, msg_iov(1)=[{"(...
     * 40 bytes in buffer 0
     | 00000  28 00 00 00 10 00 05 00  d0 d9 aa 53 00 00 00 00  (..........S.... |
     | 00010  00 00 00 00 02 00 00 00  00 00 00 00 00 00 00 00  ................ |
     | 00020  08 00 04 00 dd 05 00 00                           ........         |
    ...

    $ ./strace -e read=all ip link show > /dev/null
    recvmsg(3, {msg_name(12)={sa_family=AF_NETLINK, pid=0, groups=00000000}, msg_iov(1)=[{"...
     * 8192 bytes in buffer 0
     | 00000  34 00 00 00 02 00 00 00  00 00 00 00 ff 23 00 00  4............#.. |
     | 00010  ff ff ff ff 20 00 00 00  10 00 05 00 00 00 00 00  .... ........... |
    ...

* defs.h (dumpiov_in_msghdr): New prototype.
* net.c (extractmsghdr): New function derived from printmsghdr.
(printmsghdr): Use extractmsghdr.
(dumpiov_in_msghdr): New function.
* syscall.c (dumpio) [HAVE_SENDMSG]: Call dumpiov_in_msghdr for recvmsg
and sendmsg syscalls.

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