AK: Provide is_errno for Kernel Errors

It wouldn't make much sense on its own (as the Kernel only has errno
Errors), but it's an easy fix for not having to ifdef away every single
usage of `is_errno` in code that is shared between Userland and Kernel.

AK/Error.h

@@ -76,11 +76,11 @@ public:
     }
 
     int code() const { return m_code; }
-#ifndef KERNEL
     bool is_errno() const
     {
         return m_code != 0;
     }
+#ifndef KERNEL
     bool is_syscall() const
     {
         return m_syscall;

AK/Stream.cpp

@@ -20,15 +20,10 @@ ErrorOr<void> Stream::read_entire_buffer(Bytes buffer)
 
         auto result = read(buffer.slice(nread));
         if (result.is_error()) {
-#ifdef KERNEL
-            if (result.error().code() == EINTR) {
-                continue;
-            }
-#else
             if (result.error().is_errno() && result.error().code() == EINTR) {
                 continue;
             }
-#endif
+
             return result.release_error();
         }
 
@@ -89,15 +84,10 @@ ErrorOr<void> Stream::write_entire_buffer(ReadonlyBytes buffer)
     while (nwritten < buffer.size()) {
         auto result = write(buffer.slice(nwritten));
         if (result.is_error()) {
-#ifdef KERNEL
-            if (result.error().code() == EINTR) {
-                continue;
-            }
-#else
             if (result.error().is_errno() && result.error().code() == EINTR) {
                 continue;
             }
-#endif
+
             return result.release_error();
         }