Add ET_DYN support for the ELF loader, which required adding

relocations, add some more functions to libxunil, use include inside of
tests instead of manually defining types and add basic libc header
files. Move libxunil to the user folder, and add helloworld and
doomgeneric as apps

user/libxunil/include/stdlib.h

@@ -0,0 +1,20 @@
+#pragma once
+#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);