lib: add command to test prefix-list match

lib: add command to test prefix-list match

While we do have `show ip prefix-list NAME A.B.C.D/M`, that doesn't
actually run the prefix list matching code.  While the result would
hopefully be the same anyway, let's have a way to call the actual prefix
list match code and get a result.

(As an aside, this might be useful for scripting to do a quick "is this
prefix in that prefix list" check.)

Signed-off-by: David Lamparter <equinox@diac24.net>