Kernel+LibC: Turn errno codes into a strongly typed enum

..and allow implicit creation of KResult and KResultOr from ErrnoCode.
This means that kernel functions that return those types can finally
do "return EINVAL;" and it will just work.

There's a handful of functions that still deal with signed integers
that should be converted to return KResults.

Kernel/PerformanceEventBuffer.cpp

@@ -51,7 +51,7 @@ KResult PerformanceEventBuffer::append(int type, FlatPtr arg1, FlatPtr arg2)
 KResult PerformanceEventBuffer::append_with_eip_and_ebp(u32 eip, u32 ebp, int type, FlatPtr arg1, FlatPtr arg2)
 {
     if (count() >= capacity())
-        return KResult(-ENOBUFS);
+        return ENOBUFS;
 
     PerformanceEvent event;
     event.type = type;
@@ -67,7 +67,7 @@ KResult PerformanceEventBuffer::append_with_eip_and_ebp(u32 eip, u32 ebp, int ty
         event.data.free.ptr = arg1;
         break;
     default:
-        return KResult(-EINVAL);
+        return EINVAL;
     }
 
     auto current_thread = Thread::current();