mktemp: simplify file path parameter logic

Simplify the logic of computing the file path parameters (the
directory, prefix, suffix, and number of random characters) for the
temporary file created by `mktemp`. This commits adds an `Options`
struct as a layer of indirection between the application logic and
`clap`, and a `Params` struct whose associated function is responsible
for determining the file path parameters from the `Options`. This is
an improvement because the previous code had some logic for
determining file path parameters in one place and some in another
place.

tests/by-util/test_mktemp.rs

@@ -519,6 +519,7 @@ fn test_directory_permissions() {
 /// Test that a template with a path separator is invalid.
 #[test]
 fn test_template_path_separator() {
+    #[cfg(not(windows))]
     new_ucmd!()
         .args(&["-t", "a/bXXX"])
         .fails()
@@ -526,6 +527,14 @@ fn test_template_path_separator() {
             "mktemp: invalid template, {}, contains directory separator\n",
             "a/bXXX".quote()
         ));
+    #[cfg(windows)]
+    new_ucmd!()
+        .args(&["-t", r"a\bXXX"])
+        .fails()
+        .stderr_only(format!(
+            "mktemp: invalid template, {}, contains directory separator\n",
+            r"a\bXXX".quote()
+        ));
 }
 
 /// Test that a suffix with a path separator is invalid.