POSIX dictates here that incomplete conversions, like in
printf %d\n 15.2
or
printf %d 14g
are still printed along with any error.
This seems alright, as it allows users to silence stderr to accept incomplete conversions.
This commit implements it, but what's a bit weird is the ordering between stdout and stderr,
causing the error to be printed _after_, like
15
14
15.1: value not completely converted
14,2: value not completely converted
but that seems like a general issue with how we buffer the streams.
(I know that nonfatal_error is a copy of most of fatal_error - I tried
differently, and va_* is weird)
Fixes#5532.
This removes the caret as a shorthand for redirecting stderr.
Note that stderr may be redirected to a file via 2>/some/path...
and may be redirected with a pipe via 2>|.
Fixes#4394
Another dev noticed that tests/printf.in was failing because they didn't have
the fr_FR.UTF-8 locale installed. Make that test more resilient by trying
other locales and if no suitable locale is found skipping the test.
