Use extended errors for err(3) and warn(3)

[kib]

Summary

uexterr_format(): simplify output when ext error string is available

If the extended error string is provided by kernel, return only the
string, which is supposedly enough to identify exact cause of the error.
If the string is not provided, print the technically looking gibberish
which still allows to identify location with kernel sources.


err(3): print extended error if available

[asomers]

lib/libc/gen/err.c

96

Should this be false, since the errno is coming from the caller instead of from the kernel?

[kib]

lib/libc/gen/err.c

96

I am not sure. There are syscalls returning error instead of setting errno, and errc() is naturally used with them. I definitely mean this.

The question is probably should warnc() and errc() be consistent in this regard.