Avoid double-closing on fcntl failures (#2409)

Motivation:

The fix provided in #2407 was subtly wrong. ignoreSIGPIPE, which throws
the error in question, closes the FD on error _except_ on EINVAL from
fcntl, where it instead does not. This inconsistent behaviour is the
source of the bug. Because this behaviour is inconsistent, the fix from
PR #2407 is also inconsistent and can in some cases double-close the
socket.

The actual issue is not as old as I expected: the code can be observed
by reviewing the change in #1598, which incorrectly inserted the error
transformation before the call to close.

Modifications:

- Revert the change from #2407.
- Move the close in ignoreSIGPIPE to before the error check, rather than
  after, so we unconditionally execute it.

Result:

More resilient fix.

Sources/NIOPosix/ServerSocket.swift

@@ -107,14 +107,7 @@ import NIOCore
                 return nil
             }
 
-            let sock: Socket
-            do {
-                sock = try Socket(socket: fd)
-            } catch {
-                // best effort
-                try? Posix.close(descriptor: fd)
-                throw error
-            }
+            let sock = try Socket(socket: fd)
 
             #if !os(Linux)
             if setNonBlocking {

Sources/NIOPosix/SocketProtocols.swift

@@ -93,11 +93,11 @@ extension BaseSocketProtocol {
         do {
             try Posix.fcntl(descriptor: fd, command: F_SETNOSIGPIPE, value: 1)
         } catch let error as IOError {
+            try? Posix.close(descriptor: fd) // don't care about failure here
             if error.errnoCode == EINVAL {
                 // Darwin seems to sometimes do this despite the docs claiming it can't happen
                 throw NIOFcntlFailedError()
             }
-            try? Posix.close(descriptor: fd) // don't care about failure here
             throw error
         }
         #endif