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

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

test/read_mixer.cxx

@@ -33,7 +33,7 @@
 #include <stdlib.h>
 
 const FilterPlugin *
-filter_plugin_by_name(gcc_unused const char *name)
+filter_plugin_by_name(gcc_unused const char *name) noexcept
 {
 	assert(false);
 	return NULL;