convert narrow stderr output to wide forms

On some platforms, notably GNU libc, you cannot mix narrow and wide
stdio functions on a stream like stdout or stderr. Doing so will drop
the output of one or the other. This change makes all output to the
stderr stream consistently use the wide forms.

This change also converts some fprintf(stderr,...) calls to debug()
calls where appropriate.

Fixes #3692

src/input.cpp

@@ -374,7 +374,7 @@ int input_init() {
         } else {
             debug(0, _(L"Using fallback terminal type '%ls'"), DEFAULT_TERM);
         }
-        putc('\n', stderr);
+        fputwc(L'\n', stderr);
     }
 
     input_terminfo_init();