Make Doom run by improving and fixing libc stubs, adding proper heap

allocation, adding a timer, a framebuffer swap, rectangle and buffer
draw syscall, moving the header files to the correct directory,
disabling stack protection, AVX and MMX, while enabling SSE, fix
interrupt handler popping in the wrong order, improve
load_segment_to_memory alignment, add load_from_ptr to framebuffer for
very quick drawing, serial_print as well as render font in
consolewriter, zero the memory in sbrk, add a fake file system and
methods to libxunil for DOOM, add correct printf, vsnprintf add _start
which calls the main function in libxunil, mark all registers as used
inside syscalls for no memory corruption, add build_helloworld.sh script

user/libxunil/include/string.h

@@ -0,0 +1,22 @@
+#pragma once
+#include <stddef.h>
+
+void *memset(void *s, int c, size_t n);
+void *memcpy(void *dst, const void *src, size_t n);
+void *memmove(void *dst, const void *src, size_t n);
+int   memcmp(const void *s1, const void *s2, size_t n);
+void *memchr(const void *s, int c, size_t n);
+
+size_t strlen(const char *s);
+char  *strcpy(char *dst, const char *src);
+char  *strncpy(char *dst, const char *src, size_t n);
+char  *strcat(char *dst, const char *src);
+char  *strncat(char *dst, const char *src, size_t n);
+int    strcmp(const char *s1, const char *s2);
+int    strncmp(const char *s1, const char *s2, size_t n);
+char  *strchr(const char *s, int c);
+char  *strrchr(const char *s, int c);
+char  *strstr(const char *haystack, const char *needle);
+char  *strtok(char *str, const char *delim);
+char  *strdup(const char *s);
+char  *strerror(int errnum);