Add a scheduler which just keeps of processes for now, and a

with_process to interact with them. User space now has it's own address
space and mapper which means we will be able to allocate memory for it.
I added a bunch of functions as stubs into libxunil which are required
for doomgeneric.

user/libxunil/include/stdlib.h

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