netlink: initialize VNET context with VNET_SYSINIT()

With the initial check-in netlink(4) was very conservative with regards to
using memory and intrusiveness to the kernel and network stack. In
particular it would initialize the VNET context only on the first actuall
call to socket(PF_NETLINK), saving on allocation of a struct nl_control of
size 224 bytes.

Now it is clear that netlink(4) is primary citizen of FreeBSD, with a set
of system tools using it. So resort to normal VNET_SYSINIT() and with
that shave a lot of complexity, since after the change V_nl_ctl is
immutable.