replace every call to toupper with one to our own tolower_ascii

replace every call to toupper with one to our own tolower_ascii

This sounds like a premature optimization and since Mr. Knuth we all
know that they are the root of all evil - but, and here it starts to be
interesting: As the tolower_ascii method is by far the most called
method we have (~60 Mio. times) and as we compare only strings containing
ascii characters (package names, configuration options) using our own
method reduces execution time of APT by 4% plus it avoids that the
locale settings can influence us.