touch: support obsolete POSIX timestamp argument

Support obsolete form of timestamp argument for old POSIX versions. In
summary, when older versions of POSIX are used and the first positional
argument looks like a date and time, then treat it as a timestamp
instead of as a filename. For example, before this commit

    _POSIX2_VERSION=199209
    POSIXLY_CORRECT=1
    touch 01010000 11111111

would create two files, `01010000` and `11111111`. After this commit,
the first argument is interpreted as a date and time (in this case,
midnight on January 1 of the current year) and that date and time are
set on the file named `11111111`.

Fixes #7180.

tests/by-util/test_touch.rs

@@ -917,3 +917,27 @@ fn test_touch_reference_symlink_with_no_deref() {
     // Times should be taken from the symlink, not the destination
     assert_eq!((time, time), get_symlink_times(&at, arg));
 }
+
+#[test]
+fn test_obsolete_posix_format() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    ucmd.env("_POSIX2_VERSION", "199209")
+        .env("POSIXLY_CORRECT", "1")
+        .args(&["01010000", "11111111"])
+        .succeeds()
+        .no_output();
+    assert!(at.file_exists("11111111"));
+    assert!(!at.file_exists("01010000"));
+}
+
+#[test]
+fn test_obsolete_posix_format_with_year() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    ucmd.env("_POSIX2_VERSION", "199209")
+        .env("POSIXLY_CORRECT", "1")
+        .args(&["9001010000", "11111111"])
+        .succeeds()
+        .no_output();
+    assert!(at.file_exists("11111111"));
+    assert!(!at.file_exists("01010000"));
+}