Remove use of locale-specific strerror_r (#440)

strerror_r doesn't necessarily return UTF-8 but we assume it does.

strerror_r is locale-sensitive but we are better off not being
locale-sensitive.

strerror_r requires us to use libc but increasingly we want to avoid
libc, e.g. to support x86_64-unknown-linux-none.

Just remove the use of the function.

src/error.rs

@@ -1,3 +1,6 @@
+#[cfg(feature = "std")]
+extern crate std;
+
 use core::{fmt, num::NonZeroU32};
 
 /// A small and `no_std` compatible error type
@@ -98,35 +101,13 @@ impl Error {
     }
 }
 
-cfg_if! {
-    if #[cfg(unix)] {
-        fn os_err(errno: i32, buf: &mut [u8]) -> Option<&str> {
-            let buf_ptr = buf.as_mut_ptr().cast::<libc::c_char>();
-            if unsafe { libc::strerror_r(errno, buf_ptr, buf.len()) } != 0 {
-                return None;
-            }
-
-            // Take up to trailing null byte
-            let n = buf.len();
-            let idx = buf.iter().position(|&b| b == 0).unwrap_or(n);
-            core::str::from_utf8(&buf[..idx]).ok()
-        }
-    } else {
-        fn os_err(_errno: i32, _buf: &mut [u8]) -> Option<&str> {
-            None
-        }
-    }
-}
-
 impl fmt::Debug for Error {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let mut dbg = f.debug_struct("Error");
         if let Some(errno) = self.raw_os_error() {
             dbg.field("os_error", &errno);
-            let mut buf = [0u8; 128];
-            if let Some(err) = os_err(errno, &mut buf) {
-                dbg.field("description", &err);
-            }
+            #[cfg(feature = "std")]
+            dbg.field("description", &std::io::Error::from_raw_os_error(errno));
         } else if let Some(desc) = internal_desc(*self) {
             dbg.field("internal_code", &self.0.get());
             dbg.field("description", &desc);
@@ -140,10 +121,12 @@ impl fmt::Debug for Error {
 impl fmt::Display for Error {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         if let Some(errno) = self.raw_os_error() {
-            let mut buf = [0u8; 128];
-            match os_err(errno, &mut buf) {
-                Some(err) => err.fmt(f),
-                None => write!(f, "OS Error: {}", errno),
+            cfg_if! {
+                if #[cfg(feature = "std")] {
+                    std::io::Error::from_raw_os_error(errno).fmt(f)
+                } else {
+                    write!(f, "OS Error: {}", errno)
+                }
             }
         } else if let Some(desc) = internal_desc(*self) {
             f.write_str(desc)