config/Data: cast to std::chrono::steady_clock::duration properly

Oh no, 3413d1bf23 was broken!  Instead of passing a number as
"seconds" to the duration constructor, it just abused the duration
constructor as cast operator, which caused custom state_file_interval
settings to be extremely short.

src/config/Parser.cxx

@@ -135,3 +135,11 @@ ParseSize(const char *s, std::size_t default_factor)
 
 	return value;
 }
+
+std::chrono::steady_clock::duration
+ParseDuration(const char *s)
+{
+	// TODO: allow unit suffixes
+	const std::chrono::seconds seconds(ParseLong(s));
+	return std::chrono::duration_cast<std::chrono::steady_clock::duration>(seconds);
+}