tmpfiles: Add specifiers to allow running as user instance

This commit adds specifiers %U, %u and %h for the user UID, name and
home directory, respectively.

[zj: drop untrue copy-pasted comments and move the next text
     to the new "Specifiers" section.
     Now that #7444 has been merged, also drop the specifier functions.]

src/tmpfiles/tmpfiles.c

@@ -174,6 +174,10 @@ static const Specifier specifier_table[] = {
         { 'b', specifier_boot_id, NULL },
         { 'H', specifier_host_name, NULL },
         { 'v', specifier_kernel_release, NULL },
+
+        { 'U', specifier_user_id, NULL },
+        { 'u', specifier_user_name, NULL },
+        { 'h', specifier_user_home, NULL },
         {}
 };