ah... kunna, not kenna, whole dufferent story :) kenna has lost its 'know'
meaning, but kunna hasn't, although it's still not the same thing as vita

kunna means to know how to, kunna við means like, or be content with, and
kunna sig means to be polite

so if the sentence you had were put into modern icelandic, sá er spakur sem
sig kann, it'd mean 'he is wise(well, actually, complacent, nowadays, spakur
changed meaning too) who knows how to behave'... but kunna could in old
icelandic mean understand, know apart, have knowlege of , to be able to or
to be content with something... so i'm not saying it doesn't mean 'he who
knows himself is wise'... it just made a word-joke in my head :p

berglaug