treewide: refactor pref(erred) to preferred_lt (lifetime)

treewide: refactor pref(erred) to preferred_lt (lifetime)

Refactor "preferred" (preferred what?) and "pref" (obscure) to
"preferred_lt".

It is now more difficult to conflate prefix, preference and other "pref"
related terminology with preferred_lifetime.

Signed-off-by: Paul Donald <newtwen@gmail.com>
Reviewed-by: Daniel Golle <daniel@makrotopia.org>
[ improve commit title ]
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>