log: default debug log path to empty; skip set when empty

libfuse previously initialized its debug log filepath to
"/dev/stderr", forcing the config layer to reopen stderr on every
load whether or not the user asked for it. Default the libfuse log
path to "" and have the mergerfs config layer only forward the
request to libfuse when the user supplied an explicit path.

src/config_log_file.cpp

@@ -26,7 +26,8 @@
 
 LogFile::LogFile(const std::string &s_)
 {
-  fuse_debug_set_output(s_);
+  if(!s_.empty())
+    fuse_debug_set_output(s_);
 }
 
 std::string

vendored/libfuse/lib/debug.cpp

@@ -63,7 +63,7 @@ _debug_set_output_stderr()
   auto new_log_file = std::shared_ptr<FILE>(stderr,[](FILE*){});
 
   fuse_cfg.log_file(new_log_file);
-  fuse_cfg.log_filepath("/dev/stderr");
+  fuse_cfg.log_filepath("");
 
   return 0;
 }