o In cover.cc, the int-result of mkstemps() was assigned to a string
and silently interpreted as a single-character filename with a funny
value. Fix with the intent: assign the filename.
o in libparse.cc, an int was assigned to a string, but depending on
visible constructors, this is ambiguous. Explicitly cast this to
a char.
o Not all derived methods were marked 'override', but it is a great
feature of C++11 that we should make use of.
o While at it: touched header files got a -*- c++ -*- for emacs to
provide support for that language.
o use YS_OVERRIDE for all override keywords (though we should probably
use the plain keyword going forward now that C++11 is established)