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/from_string.cpp

@@ -16,12 +16,11 @@
   OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */
 
+#include "from_string.hpp"
+
 #include "ef.hpp"
 #include "errno.hpp"
 
-#include <cstdint>
-#include <string>
-
 #include <stdlib.h>
 
 
@@ -128,4 +127,13 @@ namespace str
   {
     return -EINVAL;
   }
+
+  int
+  from(const std::string &value_,
+       fs::Path          *path_)
+  {
+    *path_ = value_;
+
+    return 0;
+  }
 }