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

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

src/command/NeighborCommands.cxx

@@ -30,7 +30,7 @@
 #include <string>
 
 bool
-neighbor_commands_available(const Instance &instance)
+neighbor_commands_available(const Instance &instance) noexcept
 {
 	return instance.neighbors != nullptr;
 }