Avoid using a bogus CLAT address if no EUI-64

get_clat_v6_addr() didn't actually check if it saw any Modified EUI-64
based addresses on the PLAT device before proceeding. This caused it to
return a bogus CLAT address (::c1:a700:0) instead of failing with a
useful error message.
Fix truncated comment documenting readconf()
Reflow a couple of lines to fit within 80 chars
Fix a misspelling
Minor spelling and punctuation fixes