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/inttypes.h

@@ -0,0 +1 @@
+#include <stdint.h>

user/libxunil/include/stdio.h

@@ -0,0 +1,44 @@
+#pragma once
+#include <sys/types.h>
+#include <stddef.h>
+#include <stdarg.h>
+
+typedef struct FILE FILE;
+
+extern FILE *stdin;
+extern FILE *stdout;
+extern FILE *stderr;
+
+#define EOF (-1)
+
+#define SEEK_SET 0
+#define SEEK_CUR 1
+#define SEEK_END 2
+
+FILE  *fopen(const char *path, const char *mode);
+int    fclose(FILE *fp);
+size_t fread(void *ptr, size_t size, size_t nmemb, FILE *fp);
+size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *fp);
+int    fseek(FILE *fp, long offset, int whence);
+long   ftell(FILE *fp);
+int    fflush(FILE *fp);
+char  *fgets(char *s, int size, FILE *fp);
+int    fputs(const char *s, FILE *fp);
+int    feof(FILE *fp);
+int    ferror(FILE *fp);
+int    remove(const char *path);
+int    rename(const char *path, const char *new_path);
+
+int    printf(const char *fmt, ...);
+int    puts(const char *s);
+int    putchar(int c);
+
+int    fprintf(FILE *fp, const char *fmt, ...);
+int    sprintf(char *buf, const char *fmt, ...);
+int    snprintf(char *buf, size_t size, const char *fmt, ...);
+int    vsnprintf(char *buf, size_t size, const char *fmt, va_list ap);
+int    vfprintf(FILE *fp, const char *fmt, va_list ap);
+ssize_t write(int fd, const void *buf, size_t count);
+void exit(int code);
+
+int    sscanf(const char *str, const char *format, ...);

user/libxunil/include/stdlib.h

@@ -0,0 +1,24 @@
+#pragma once
+#include <inttypes.h>
+#include <stddef.h>
+
+void  *malloc(size_t size);
+void  *calloc(size_t nmemb, size_t size);
+void  *realloc(void *ptr, size_t size);
+void   free(void *ptr);
+
+void   exit(int status);
+void   abort(void);
+
+int    atoi(const char *s);
+long   atol(const char *s);
+double atof(const char *s);
+long   strtol(const char *s, char **endptr, int base);
+double strtod(const char *s, char **endptr);
+
+char  *getenv(const char *name);
+void   qsort(void *base, size_t nmemb, size_t size, int (*cmp)(const void *, const void *));
+int    abs(int x);
+int    system(const char *cmd);
+int    draw_pixel(uint32_t x, uint32_t y, uint32_t color);
+int    framebuffer_swap();

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);

user/libxunil/include/strings.h

@@ -0,0 +1,8 @@
+#pragma once
+#include <stddef.h>
+
+int    strcasecmp(const char *s1, const char *s2);
+int    strncasecmp(const char *s1, const char *s2, size_t n);
+void   bzero(void *s, size_t n);
+void   bcopy(const void *src, void *dest, size_t n);
+int    bcmp(const void *s1, const void *s2, size_t n);