utf8: non-character U+fffe as invalid codepoint

Unicode non-characters [1] are guaranteed to never be used for a
character. The formerly used U+fffd however is a valid character - the
replacement character [2] correctly displayed by Qt5 as <?>.

[1] https://en.wikipedia.org/wiki/Universal_Character_Set_characters#Non-characters
[2] https://en.wikipedia.org/wiki/Specials_(Unicode_block)#Replacement_character

Issue #3483

repos/gems/src/server/terminal/main.cc

@@ -368,7 +368,7 @@ void Terminal::Main::_handle_input()
 
 			if (special_sequence)
 				_read_buffer.add(special_sequence);
-			else
+			else if (codepoint.valid())
 				_read_buffer.add(codepoint);
 		});
 	});

repos/os/include/util/utf8.h

@@ -25,7 +25,7 @@ namespace Genode {
 
 struct Genode::Codepoint
 {
-	static constexpr uint32_t INVALID = 0xfffd;
+	static constexpr uint32_t INVALID = 0xfffe;
 
 	uint32_t value;
 

repos/os/src/test/utf8/main.cc

@@ -107,7 +107,7 @@ void Component::construct(Genode::Env &env)
 	              Expect_codepoint(0x3b5));
 
 	/* 2.1.1  1 byte  (U-00000000) */
-	test("<EFBFBD>", Expect_invalid(), Expect_end());
+	test("\x00", Expect_codepoint(0x00), Expect_end());
 
 	/* 2.1.2  2 bytes (U-00000080) */
 	test("\xc2\x80", Expect_codepoint(0x80), Expect_end());