cat: report copy_file_range() errors with the read filename

The error may be due to an error while writing, but it's more helpful to
report what we were trying to from rather than "stdout" for these
errors.  Noticed because of what happens now when `cat` gets an EISDIR
for reading from a dirfd:

kevans@ifrit:~$ cat /etc
cat: stdout: Is a directory

which, after this change, becomes:

kevans@aarch64-dev:~$ cat /etc
cat: /etc: Is a directory

Reviewed by:	allanjude, des, mm
Differential Revision:	https://reviews.freebsd.org/D41784

bin/cat/cat.c

@@ -271,7 +271,7 @@ scanfiles(char *argv[], int cooked __unused)
 				    errno == EISDIR)
 					raw_cat(fd);
 				else
-					err(1, "stdout");
+					err(1, "%s", filename);
 			}
 #else
 			raw_cat(fd);