*: add "noexcept" to many, many function prototypes

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

src/IOThread.hxx

@@ -51,13 +51,13 @@ io_thread_deinit();
 
 gcc_const
 EventLoop &
-io_thread_get();
+io_thread_get() noexcept;
 
 /**
  * Is the current thread the I/O thread?
  */
 gcc_pure
 bool
-io_thread_inside();
+io_thread_inside() noexcept;
 
 #endif