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

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

src/fs/Charset.hxx

@@ -33,7 +33,7 @@
  */
 gcc_const
 const char *
-GetFSCharset();
+GetFSCharset() noexcept;
 
 /**
  * Throws std::runtime_error on error.
@@ -42,7 +42,7 @@ void
 SetFSCharset(const char *charset);
 
 void
-DeinitFSCharset();
+DeinitFSCharset() noexcept;
 
 /**
  * Convert the path to UTF-8.