Added screen management functionalities, string manipulation
and string output.
Fix strtol sign and handling of non base member
Added printk and many improvements

- added printk and vprintk
- improved number formating for itoa
- putchar is a print module function
- mimic write(2) for the screen

Construction:
    Since gcc and clang no longer recognize the cdecl attribute
and use a register-base call in their abi, it is impossible
to implement a self-made stdarg for the kernel.
Therefore, the -nostdinc option have been removed from the
construction process in order to let the compiler use its
builtin stdarg management.
Added strncat to string.h