net: ip: Add iovec and msghdr structs
net: ip: Add iovec and msghdr structs

These are needed in sendmsg implementation. See RFC 3542 for
details of the msghdr struct and the macros that are used
to manipulate it.

Signed-off-by: Jukka Rissanen <f9f077d1da4c7aa947f9864fa5a28c3588f6a45d@linux.intel.com>

finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,galak/zephyr