Implement sendmmsg syscall decoder

Implement sendmmsg syscall decoder

* linux/dummy.h (sys_sendmmsg): Remove.
* linux/syscall.h (sys_sendmmsg): New prototype.
* net.c (printmmsghdr): Add index argument specifying the element in
mmsghdr array to print.
(decode_mmsg): New function, prints the whole mmsghdr array, its length
and message flags.
(sys_sendmmsg): New function.
(sys_recvmmsg): Use decode_mmsg to fix mmsghdr array decoding.