Remove Errno consts from platforms where they aren't defined.

Previously these were aliases for UnknownErrno.  Now, they're compile
errors.

Author: asomers

CHANGELOG.md

@@ -57,6 +57,8 @@ This project adheres to [Semantic Versioning](https://semver.org/).
   longer be needed now that async/await are available.  `AioCb`s now work
   exclusively with borrowed buffers, not owned ones.
   (#[1440](https://github.com/nix-rust/nix/pull/1440))
+- Removed some Errno values from platforms where they aren't actually defined.
+  (#[1452](https://github.com/nix-rust/nix/pull/1452))
 
 ## [0.20.0] - 20 February 2021
 ### Added

src/errno.rs

@@ -1107,8 +1107,6 @@ mod consts {
     pub const EWOULDBLOCK: Errno = Errno::EAGAIN;
     pub const EDEADLOCK:   Errno = Errno::EDEADLK;
 
-    pub const EL2NSYNC: Errno = Errno::UnknownErrno;
-
     pub fn from_i32(e: i32) -> Errno {
         use self::Errno::*;
 
@@ -1332,8 +1330,6 @@ mod consts {
     pub const EWOULDBLOCK: Errno = Errno::EAGAIN;
     pub const EDEADLOCK:   Errno = Errno::EDEADLK;
 
-    pub const EL2NSYNC: Errno = Errno::UnknownErrno;
-
     pub fn from_i32(e: i32) -> Errno {
         use self::Errno::*;
 
@@ -1547,8 +1543,6 @@ mod consts {
     pub const EDEADLOCK:   Errno = Errno::EDEADLK;
     pub const EOPNOTSUPP:  Errno = Errno::ENOTSUP;
 
-    pub const EL2NSYNC: Errno = Errno::UnknownErrno;
-
     pub fn from_i32(e: i32) -> Errno {
         use self::Errno::*;
 
@@ -1759,8 +1753,6 @@ mod consts {
     pub const ELAST: Errno       = Errno::ENOTSUP;
     pub const EWOULDBLOCK: Errno = Errno::EAGAIN;
 
-    pub const EL2NSYNC: Errno = Errno::UnknownErrno;
-
     pub fn from_i32(e: i32) -> Errno {
         use self::Errno::*;
 
@@ -1972,8 +1964,6 @@ mod consts {
     pub const ELAST: Errno       = Errno::ENOTSUP;
     pub const EWOULDBLOCK: Errno = Errno::EAGAIN;
 
-    pub const EL2NSYNC: Errno = Errno::UnknownErrno;
-
     pub fn from_i32(e: i32) -> Errno {
         use self::Errno::*;
 
@@ -2172,11 +2162,8 @@ mod consts {
         EPROTO = libc::EPROTO,
     }
 
-    pub const ELAST: Errno = Errno::UnknownErrno;
     pub const EWOULDBLOCK: Errno = Errno::EAGAIN;
 
-    pub const EL2NSYNC: Errno = Errno::UnknownErrno;
-
     pub fn from_i32(e: i32) -> Errno {
         use self::Errno::*;