(FACT-680) Correctly detect ipv6 link-local addresses

The regex in lib/facter/ipaddress6.rb will incorrectly detect all
addresses with 'fe80' in them and identify them as link-local addresses.

Should only occur when addresses start with fe80:

Modified existing tests to include an address with fe80 in it.

spec tests
Merge pull request #786 from obliadp/ipv6-linklocal

(FACT-680) Correctly detect ipv6 link-local addresses