linux-info.eclass: Die in most of public-ish APIs on non-Linux

linux-info.eclass: Die in most of public-ish APIs on non-Linux

Add appropriate 'die' calls in most of the seemingly public APIs
of the eclass that could be called by ebuilds and that are going to fail
horribly when used on non-Linux systems. This means that
e.g. 'kernel_is' calls need to be explicitly guarded in ebuilds, as we
can't really reasonably return 'true' or 'false' if there is no Linux
kernel in the first place.