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 <[email protected]>
Reviewed-by: Daniel Golle <[email protected]>
[ improve commit title ]
Signed-off-by: Christian Marangi <[email protected]>