Don't rely on integer wrapping in ctype and fix strcasecmp on
non-alphabetic characters

Implemented the following calls according to http://pubs.opengroup.org/onlinepubs/7908799/xsh/libgen.h.html
- char* basename(char*)
- char* dirname(char*)

Added test suit for the implemented calls.

Issue: redox-os/relibc#134

Based on what musl does.

wow i am stupid for writing this code

Bug discovered by @xTibor. Test and input data provided by him.

Apparently gmtime was already implemented when I made localtime, so we had two different things written from scratch. We decided in the relibc channel of the Redox OS Mattermost chat to use my code, as it is more extensively tested and perhaps is clearer in how it works.