IPv6: Track Interface: Doesn't allow the full range of IPv6 Prefix id's

[lrosenman]

I get a /60 from ATT, which should allow prefix id's 0-15 (4 bits).  The GUI for track interface says 15 & 14 are out of bounds.

This is in addition to the problem of adding a new interface that tracks WAN *NOT* rewriting the dhcp6c_<WAN>.conf file and restarting dhcp6c.

[lrosenman]

This is solved, but the documentation or tool-tips should show that it is a HEX number (0-f) in this case.