POSIX: don't force-unwrap result of fdopendir(3)

Motivation:

It's possible for fdopendir(3) to return 0 if the file descriptor's backing
filesystem is force-unmounted. This should return an error, but instead crashes
the library.

Result:

If fdopendir fails, FileDescriptor.opendir will return the error contained
in errno (set by the stat(2) call in libc fdopendir).

Author: recursiveforte

Sources/NIOFileSystem/Internal/System Calls/Errno.swift

@@ -146,3 +146,25 @@ public func valueOrErrno<R>(
         }
     }
 }
+
+
+/// As `valueOrErrno` but returns `Errno` if result is nil 
+@_spi(Testing)
+public func unwrapValueOrErrno<R>(
+    retryOnInterrupt: Bool = true,
+    _ fn: () -> R?
+) -> Result<R, Errno> {
+    while true {
+        Errno.clear()
+        if let result = fn() {
+            return .success(result)
+        } else {
+            let errno = Errno._current
+            if errno == .interrupted, retryOnInterrupt {
+                continue
+            } else {
+                return .failure(errno)
+            }
+        }
+    }
+}

Sources/NIOFileSystem/Internal/System Calls/FileDescriptor+Syscalls.swift

@@ -191,7 +191,7 @@ extension FileDescriptor {
     ///     caller should not modify the descriptor or close the descriptor via `close()`. Once
     ///     directory iteration has been completed then `Libc.closdir(_:)` must be called.
     internal func opendir() -> Result<CInterop.DirPointer, Errno> {
-        valueOrErrno(retryOnInterrupt: false) {
+        unwrapValueOrErrno(retryOnInterrupt: false) {
             libc_fdopendir(self.rawValue)
         }
     }

Sources/NIOFileSystem/Internal/System Calls/Syscalls.swift

@@ -368,8 +368,8 @@ internal func system_futimens(
 /// fdopendir(3): Opens a directory stream for the file descriptor
 internal func libc_fdopendir(
     _ fd: FileDescriptor.RawValue
-) -> CInterop.DirPointer {
-    fdopendir(fd)!
+) -> CInterop.DirPointer? {
+    fdopendir(fd)
 }
 
 /// readdir(3): Returns a pointer to the next directory entry