Use safer rstime_t instead of time_t

Avoid problems to serialization on different platforms, without breaking
nested STL containers serialization.

The conversion have been made with sed, and checked with grep, plus
kdiff3 visual ispection, plus rutime tests, so it should be fine.

libretroshare/src/file_sharing/README.txt

@@ -90,10 +90,10 @@ Classes
       - std::string   name
       - RsFileHash    hash
       - uint64_t      size
-      - time_t        Last modification time
+      - rstime_t        Last modification time
 
    LocalFileInfo: public FileInfo
-      - time_t        Last data access time
+      - rstime_t        Last data access time
       - uint64_t      Total data uploaded
       - uint32_t      ShareFlags