Some F5 server configurations appear to require an IPv6 netmask in this
form.

This adds a netmasklen6() function, analogous to the Legacy IP version in
netmasklen().  It also adds gcc-optimized forms, using __builtin_clz, for
both netmasklen functions.

[dwmw2: Detect __builtin_clz() with autoconf, wrap it once in cls()]
Signed-off-by: Daniel Lenski <dlenski@gmail.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>