Update: Redesign f_file functions to not need struct stat passed and other improvements.

Update: Redesign f_file functions to not need struct stat passed and other improvements.

While working on unit tests I noticed that passing struct stat seems inconsistent.

While there is likely a performance gain of not having to call the stat() functions, the inconsistency is undesirable.

Optimizations can be done under another API series.

Add missing path.used check.