FOC: Fix 'user_flags' on x86

Use carry bit in EFLAGS to pass thread state to ex_reg caller. Do not touch
other bits, which might disable interrupts on kernel exit.

Fixes ssumpf/foc#2.

kernel/fiasco/src/kern/ia32/thread-ia32.cpp

@@ -92,7 +92,10 @@ Thread::user_ip() const
 IMPLEMENT inline
 Mword
 Thread::user_flags() const
-{ return state() & Thread_ready; }
+{
+    // pass thread-ready state in carry flag (Thread_ready is bit 0)
+    return (regs()->flags() & ~Thread_ready) | (state() & Thread_ready);
+}
 
 /** Check if the pagefault occured at a special place: At some places in the
     kernel we want to ensure that a specific address is mapped. The regular