Improve mount waiting + misc doc improvements

Adds user.mergerfs.branch and user.mergerfs.branch_mounts_here
Adds .mergerfs.branch and .mergerfs.branch_mounts_here files.

src/to_string.cpp

@@ -16,6 +16,8 @@
   OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */
 
+#include "to_string.hpp"
+
 #include <cstdint>
 #include <string>
 
@@ -55,4 +57,10 @@ namespace str
   {
     return s_;
   }
+
+  std::string
+  to(const fs::Path &path_)
+  {
+    return path_.string();
+  }
 }