Linux: Drop obsolete kernel support with `if_nameindex' and `if_nametoindex'

Linux: Drop obsolete kernel support with `if_nameindex' and `if_nametoindex'

Support for the SIOCGIFINDEX ioctl(2) Linux ABI (0x8933 command, called
SIOGIFINDEX in the API originally) was added with kernel version 2.1.14
for AF_INET6 sockets, followed by general support with version 2.1.22.
The Linux API was then updated by adding the current SIOCGIFINDEX name
with kernel version 2.1.68, back in Nov 1997.

All these kernel versions are well below our current default required
minimum of 3.2.0, let alone some platform higher version requirements.

Drop support for the absence of the SIOCGIFINDEX ioctl(2) in the API or
ABI, by removing arrangements for the ENOSYS error condition.  Discard
the indirection from '__if_nameindex' to 'if_nameindex_netlink' and
adjust the implementation of '__if_nametoindex' accordingly for a better
code flow.