Fix clippy reported problems of write_with_newline lint

This change fixes occurrences flagged by the clippy lint
`write_with_newline`. E.g.,

> warning: using `write!()` with a format string that ends in a single
>          newline, consider using `writeln!()` instead
>    --> src/parser.rs:410:29
>     |
> 410 | /                             write!(self.stderr,
> 411 | |                                 "WARNING: Environment variable {}: {}\n",
> 412 | |                                 evar.name, err).ok();
>     | |_______________________________________________^
>     |
>     = note: #[warn(clippy::write_with_newline)] on by default
>     = help: for further information visit
>       https://rust-lang-nursery.github.io/rust-clippy/master/index.html#write_with_newline

Author: d-e-s-o

src/parser.rs

@@ -407,8 +407,8 @@ impl<'a, 'b> Context<'a, 'b> {
                             continue;
                         }
                         Error(err) => {
-                            write!(self.stderr,
-                                "WARNING: Environment variable {}: {}\n",
+                            writeln!(self.stderr,
+                                "WARNING: Environment variable {}: {}",
                                 evar.name, err).ok();
                         }
                         _ => unreachable!(),
@@ -761,7 +761,7 @@ impl<'parser> ArgumentParser<'parser> {
     /// of scope of the argparse
     pub fn error(&self, command: &str, message: &str, writer: &mut Write) {
         self.print_usage(command, writer).unwrap();
-        write!(writer, "{}: {}\n", command, message).ok();
+        writeln!(writer, "{}: {}", command, message).ok();
     }
 
     /// Configure parser to ignore options when first non-option argument is