Kernel: Add realpath syscall

Author: https://github.com/rok-povsic
Commit: 18fbe4ac83
Pull-request: https://github.com/SerenityOS/serenity/pull/473
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/bugaevc

Kernel/Process.h

@@ -222,6 +222,7 @@ public:
     int sys$halt();
     int sys$reboot();
     int sys$set_process_icon(int icon_id);
+    int sys$realpath(const char* pathname, char*, size_t);
 
     static void initialize();