Add write_file () syscall, CE implement redirections, libarena arena_realloc ()

libsystem/system.c

@@ -70,3 +70,7 @@ int read_dir_entry (const char* path, struct dir_entry* entry, size_t entry_num)
 }
 
 int create_file (const char* path) { return (int)do_syscall (SYS_CREATE_FILE, path); }
+
+int write_file (const char* path, size_t off, uint8_t* buffer, size_t size) {
+  return (int)do_syscall (SYS_WRITE_FILE, path, off, buffer, size);
+}