save and restore segment registers

=> this fixed a bug in the system call "fork"

arch/x86/kernel/entry.asm

@@ -440,10 +440,10 @@ extern syscall_handler
 
 ; used to realize system calls
 isrsyscall:
-    ;push ds
-    ;push fs
-    ;push gs
-    ;push es
+    push ds
+    push fs
+    push gs
+    push es
     push ebp
     push edi
     push esi
@@ -460,10 +460,10 @@ isrsyscall:
     pop esi
     pop edi
     pop ebp
-    ;pop es
-    ;pop gs
-    ;pop fs
-    ;pop ds
+    pop es
+    pop gs
+    pop fs
+    pop ds
     iret
 
 global irq0