Detect windows line endings when executing a file

Fixes #2783.

tests/checks/exec.fish

@@ -9,5 +9,15 @@ not exec cat <nosuchfile
 #CHECKERR: open: No such file or directory
 echo "neg failed: $status"
 #CHECK: neg failed: 0
+
+set -l f (mktemp)
+echo "#!/bin/sh"\r\n"echo foo" > $f
+chmod +x $f
+$f
+#CHECKERR: Failed to execute process '{{.*}}'. Reason:
+#CHECKERR: The file uses windows line endings (\r\n). Run dos2unix or similar to fix it.
+
+# This needs to be last, because it actually runs exec.
 exec cat </dev/null
 echo "not reached"
+