Kernel: Make pseudo terminals not overwrite old data

If pseudo terminal buffer was filled, old implementation would overwrite
old data. This is bad if producer is capable of producing more data than
consumer can handle.

kernel/include/kernel/Terminal/TTY.h

@@ -58,14 +58,14 @@ namespace Kernel
 	protected:
 		TTY(mode_t mode, uid_t uid, gid_t gid);
 
-		virtual void putchar_impl(uint8_t ch) = 0;
+		virtual bool putchar_impl(uint8_t ch) = 0;
 		virtual void update_cursor() {}
 
 		virtual BAN::ErrorOr<size_t> read_impl(off_t, BAN::ByteSpan) override;
 		virtual BAN::ErrorOr<size_t> write_impl(off_t, BAN::ConstByteSpan) override;
 
 	private:
-		void putchar(uint8_t ch);
+		bool putchar(uint8_t ch);
 		void do_backspace();
 
 	protected: