Create logfiles in mode 0640

Patch from toralf; closes 21729.

changes/ticket21729

@@ -0,0 +1,7 @@
+  o Minor features (logging):
+    - Log files are no longer created world-readable by default.
+      (Previously, most distributors would store the logs in a
+      non-world-readable location to prevent inappropriate access. This
+      change is an extra precaution.)  Closes ticket 21729; patch from
+      toralf.
+

src/common/log.c

@@ -1086,7 +1086,7 @@ add_file_log(const log_severity_list_t *severity, const char *filename,
   int open_flags = O_WRONLY|O_CREAT;
   open_flags |= truncate_log ? O_TRUNC : O_APPEND;
 
-  fd = tor_open_cloexec(filename, open_flags, 0644);
+  fd = tor_open_cloexec(filename, open_flags, 0640);
   if (fd<0)
     return -1;
   if (tor_fd_seekend(fd)<0) {