Make more line styles work automagically

Didn't find out how to make a pull request for only one commit, but I think that the second commit (autodection of the pngcairo terminal) makes sense, anyway:
 - manually letting the user enable the terminal is better than nothing but works only for him who reads the manuals.
 - detecting the supported terminals once (and never again) leads to strange failures on system configuration changes
 - detecting the supported terminals once and automagically again on strange failures is more lisp than I am capable of at the moment
 - and autodetection is this fast that it just transparently adds a feature.