Improved kernel memory usage

Moved base linear address to 0xFF800000
Kernel only reserves the page frames it actually needs
Memory for multiboot2 headers is freed
Video memory and APIC registers are dynamically mapped into linear addresses

include/stdio.h

@@ -2,6 +2,8 @@
 
 #include <stdarg.h>
 
+int initialize_screen();
+
 int putchar(int c);
 
 int puts(const char *str);