gnrc_netif: only use prefix matching as tie-breaker in source selection
gnrc_netif: only use prefix matching as tie-breaker in source selection

When source address selection is done, both RFC and comments in the code
state, that a longest prefix match should *only* be used as a
tie-breaker between more than one viable candidate. If there is only one
address, there is

a) no need for a tie-breaker
b) in the case of either the destination address or the single remaining
   address being ULAs ([which are considered to be of global scope]
   [RFC4193]) possibly not matching, as `fd00::/7` and e.g. `2001::/8`
   do not have a common prefix.

(b) in fact causes the match function to return -1, causing the source
address selection to return -1, causing the outer function to return the
first address it found (which most often is the link-local address),
causing e.g. a ping to an ULA to fail, even is there is a global
address.

[RFC4193]: https://tools.ietf.org/html/rfc4193

RIOT-OS/RIOT,ant9000/RIOT,ant9000/RIOT,OTAkeys/RIOT,josephnoir/RIOT,yogo1212/RIOT,mtausig/RIOT,toonst/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,authmillenon/RIOT,smlng/RIOT,kYc0o/RIOT,OTAkeys/RIOT,kaspar030/RIOT,toonst/RIOT,miri64/RIOT,toonst/RIOT,josephnoir/RIOT,authmillenon/RIOT,kYc0o/RIOT,basilfx/RIOT,jasonatran/RIOT,basilfx/RIOT,yogo1212/RIOT,mtausig/RIOT,smlng/RIOT,OTAkeys/RIOT,toonst/RIOT,OlegHahm/RIOT,ant9000/RIOT,jasonatran/RIOT,x3ro/RIOT,aeneby/RIOT,aeneby/RIOT,jasonatran/RIOT,x3ro/RIOT,miri64/RIOT,basilfx/RIOT,smlng/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,authmillenon/RIOT,aeneby/RIOT,miri64/RIOT,miri64/RIOT,yogo1212/RIOT,kaspar030/RIOT,josephnoir/RIOT,aeneby/RIOT,yogo1212/RIOT,kaspar030/RIOT,josephnoir/RIOT,kaspar030/RIOT,OTAkeys/RIOT,miri64/RIOT,kYc0o/RIOT,authmillenon/RIOT,mtausig/RIOT,basilfx/RIOT,jasonatran/RIOT,authmillenon/RIOT,josephnoir/RIOT,aeneby/RIOT,smlng/RIOT,OlegHahm/RIOT,x3ro/RIOT,smlng/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,x3ro/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,kYc0o/RIOT,authmillenon/RIOT,mtausig/RIOT,yogo1212/RIOT,yogo1212/RIOT,jasonatran/RIOT,x3ro/RIOT,ant9000/RIOT,toonst/RIOT,ant9000/RIOT,mtausig/RIOT,RIOT-OS/RIOT,basilfx/RIOT