Not retry calling close(...) when EINTR is reported. (#217)

Motivation:

Usually we retry the syscall when EINTR is reported. This is not safe in the case of close(...) as the file descriptor may already be closed. The problem arise if this was the case and we retry it and the file descriptor is already reused again. In this case we would close a wrong file descriptor.

See:
  - https://bugs.chromium.org/p/chromium/issues/detail?id=269623
  - https://lwn.net/Articles/576478/

Modifications:

Ignore EINTR on close(...) and just assume the FD was closed.

Result:

Not possible to close the wrong file descriptor when close(...) reports EINTR.

Sources/NIO/System.swift

@@ -184,8 +184,20 @@ internal enum Posix {
 
     @inline(never)
     public static func close(descriptor: CInt) throws {
-        _ = try wrapSyscall {
-            sysClose(descriptor)
+        let res = sysClose(descriptor)
+        if res == -1 {
+            let err = errno
+
+            // There is really nothing "sane" we can do when EINTR was reported on close.
+            // So just ignore it and "assume" everything is fine == we closed the file descriptor.
+            //
+            // For more details see:
+            //     - https://bugs.chromium.org/p/chromium/issues/detail?id=269623
+            //     - https://lwn.net/Articles/576478/
+            if err != EINTR {
+                assert(!isBlacklistedErrno(err), "blacklisted errno \(err) \(strerror(err)!)")
+                throw IOError(errnoCode: err, function: "close")
+            }
         }
     }