LibC: add stubs for a lot of functions

libc/include/unistd.h

@@ -1,5 +1,21 @@
 #pragma once
 
+#include <sys/types.h>
+
 #define STDIN_FILENO 0
 #define STDOUT_FILENO 1
 #define STDERR_FILENO 2
+
+// fork(), execv(), execve(), execvp(), getpid()
+
+__BEGIN_DECLS
+
+pid_t fork(void);
+
+int execv(const char*, char* const[]);
+int execve(const char*, char* const[], char* const[]);
+int execvp(const char*, char* const[]);
+
+pid_t getpid(void);
+
+__END_DECLS