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

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

src/db/Registry.hxx

@@ -32,6 +32,6 @@ extern const DatabasePlugin *const database_plugins[];
 
 gcc_pure
 const DatabasePlugin *
-GetDatabasePluginByName(const char *name);
+GetDatabasePluginByName(const char *name) noexcept;
 
 #endif