Build more cleanly with NetBSDs that dislike toupper(char)

Build more cleanly with NetBSDs that dislike toupper(char)

To be fair, when char can be signed, if toupper doesn't take negative
characters, toupper(char) is a very bad idea.  So let's just use the
nice safe EVUTIL_TOUPPER instead.  (It explicitly only upcases ASCII,
but we only use it for identifiers that we know to be ASCII anyway).