cat: fix for numbered lines w/ no trailing newline

Make at_line_start persist between printing each file. This fixes an
issue when numbering lines in the output and one of the input files
does not have a trailing newline.

tests/fixtures/cat/nonewline.txt

@@ -0,0 +1 @@
+text without a trailing newline

tests/test_cat.rs

@@ -22,6 +22,15 @@ fn test_output_multi_files_print_all_chars() {
                 pM-qM-rM-sM-tM-uM-vM-wM-xM-yM-zM-{M-|M-}M-~M-^?");
 }
 
+#[test]
+fn test_numbered_lines_no_trailing_newline() {
+    new_ucmd!()
+        .args(&["nonewline.txt", "alpha.txt", "-n"])
+        .succeeds()
+        .stdout_only("     1\ttext without a trailing newlineabcde\n     2\tfghij\n     \
+                3\tklmno\n     4\tpqrst\n     5\tuvwxyz\n");
+}
+
 #[test]
 fn test_stdin_show_nonprinting() {
     for same_param in vec!["-v", "--show-nonprinting"] {