true: Rework to return true more often

Now treats recognized command line options and ignores unrecognized
command line options instead of returning a special exit status for
them.

There is one point of interest, which is related to an implementation
detail in GNU `true`. It may return a non-true exit status (in
particular EXIT_FAIL) if writing the diagnostics of a GNU specific
option fails. For example `true --version > /dev/full` would fail and
have exit status 1.

	This behavior was acknowledged in gnu in commit
	<9a6a486e6503520fd2581f2d3356b7149f1b225d>. No further
	justification provided for keeping this quirk.

POSIX knows no such options, and requires an exit status of 0 in all
cases. We replicate GNU here which is a consistency improvement over the
prior implementation. Adds documentation to clarify the intended
behavior more properly.

src/uu/true/src/true.rs

@@ -4,16 +4,41 @@
 //  *
 //  * For the full copyright and license information, please view the LICENSE
 //  * file that was distributed with this source code.
+use clap::{App, AppSettings, ErrorKind};
+use std::io::Write;
+use uucore::error::{set_exit_code, UResult};
 
-use clap::{App, AppSettings};
+static ABOUT: &str = "
-use uucore::error::UResult;
+ Returns true, a successful exit status.
+
+ Immediately returns with the exit status `0`, except when invoked with one of the recognized
+ options. In those cases it will try to write the help or version text. Any IO error during this
+ operation causes the program to return `1` instead.
+";
 
 #[uucore::main]
 pub fn uumain(args: impl uucore::Args) -> UResult<()> {
-    uu_app().get_matches_from(args);
+    let app = uu_app();
+
+    if let Err(err) = app.try_get_matches_from(args) {
+        if let ErrorKind::DisplayHelp | ErrorKind::DisplayVersion = err.kind {
+            if let Err(print_fail) = err.print() {
+                // Try to display this error.
+                let _ = writeln!(std::io::stderr(), "{}: {}", uucore::util_name(), print_fail);
+                // Mirror GNU options. When failing to print warnings or version flags, then we exit
+                // with FAIL. This avoids allocation some error information which may result in yet
+                // other types of failure.
+                set_exit_code(1);
+            }
+        }
+    }
+
     Ok(())
 }
 
 pub fn uu_app<'a>() -> App<'a> {
-    App::new(uucore::util_name()).setting(AppSettings::InferLongArgs)
+    App::new(uucore::util_name())
+        .version(clap::crate_version!())
+        .about(ABOUT)
+        .setting(AppSettings::InferLongArgs)
 }