DetachedSong, db/LightSong, db/simple/Song: use std::chrono::system_clock::time_point

src/db/plugins/simple/Song.cxx

@@ -29,7 +29,7 @@
 #include <string.h>
 
 inline Song::Song(const char *_uri, size_t uri_length, Directory &_parent)
-	:parent(&_parent), mtime(0),
+	:parent(&_parent), mtime(std::chrono::system_clock::time_point::min()),
 	 start_time(SongTime::zero()), end_time(SongTime::zero())
 {
 	memcpy(uri, _uri, uri_length + 1);

src/db/plugins/simple/Song.hxx

@@ -70,7 +70,11 @@ struct Song {
 	 */
 	Directory *const parent;
 
-	time_t mtime;
+	/**
+	 * The time stamp of the last file modification.  A negative
+	 * value means that this is unknown/unavailable.
+	 */
+	std::chrono::system_clock::time_point mtime;
 
 	/**
 	 * Start of this sub-song within the file.