Implement putstr and putstrn in ui_file

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.