In my tour of the ui_file subsystem, I found that fputstr and fputstrn
can be simplified. The _filtered forms are never used (and IMO
unlikely to ever be used) and so can be removed. And, the interface
can be simplified by removing a callback function and moving the
implementation directly to ui_file.
A new self-test is included. Previously, I think nothing was testing
this code.
Regression tested on x86-64 Fedora 34.
d53fd721a1