state_file: don't rewind the stream while reading the state file

Parse the state file line by line, let each subsystem probe a line.
Only the playlist_state code gets the FILE pointer to read the
following lines.

src/volume.h

@@ -31,7 +31,8 @@ int volume_level_get(void);
 
 bool volume_level_change(unsigned volume);
 
-void read_sw_volume_state(FILE *fp);
+bool
+read_sw_volume_state(const char *line);
 
 void save_sw_volume_state(FILE *fp);