iproute2: ip address add prefer keyword confusion

commit 78d04c7b27cf ("ipaddress: Add support for address metric")
added "priority" and "preference" as aliases for the "metric" keyword,
but they are entirely undocumented.

I only noticed because I was adding addresses with a preferred
lifetime, but I was using "pref" as the keyword. The metric code was
added _above_ the lifetime code, so after the change "pref" matches
"preference", instead of "preferred_lft".

Is there an existing way to deal with conflicts between keyword
prefixes? Should "prefer" (or shorter) fail with a clear error
instead? Should the metric code have been added below the lifetime
code? Should it be moved or is it too late?

-- 
- Norman Rasmussen
 - Email: norman@rasmussen.co.za
 - Home page: http://norman.rasmussen.co.za/