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/string.h

@@ -1,22 +0,0 @@
-#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);