SystemInternals: make WindowsSyscallAdapters build

Correct some type errors, missing return type.  Add a FIXME comment
regarding large file access.

Author: compnerd

Sources/SystemInternals/Exports.swift

@@ -21,6 +21,12 @@ import Glibc
 
 public typealias COffT = off_t
 
+#if os(Windows)
+public typealias CModeT = CInt
+#else
+public typealias CModeT = mode_t
+#endif
+
 // MARK: syscalls and variables
 
 #if os(macOS) || os(iOS) || os(watchOS) || os(tvOS)

Sources/SystemInternals/WindowsSyscallAdapters.swift

@@ -9,16 +9,19 @@
 
 #if os(Windows)
 
+import ucrt
+import WinSDK
+
 @inline(__always)
-internal func open(_ path: UnsafePointer<CChar>, _ oflag: Int32) {
+internal func open(_ path: UnsafePointer<CChar>, _ oflag: Int32) -> CInt {
   var fh: CInt = -1
   _ = _sopen_s(&fh, path, oflag, _SH_DENYNO, _S_IREAD | _S_IWRITE)
   return fh
 }
 
 @inline(__always)
 internal func open(
-  _ path: UnsafePointer<CChar>, _ oflag: Int32, _ mode: mode_t
+  _ path: UnsafePointer<CChar>, _ oflag: Int32, _ mode: CModeT
 ) -> CInt {
   // TODO(compnerd): Apply read/write permissions
   var fh: CInt = -1
@@ -33,9 +36,9 @@ internal func close(_ fd: Int32) -> Int32 {
 
 @inline(__always)
 internal func lseek(
-  _ fd: Int32, _ off: off_t, _ whence: Int32
-) -> off_t {
-  _lseek(fd, off, whence)
+  _ fd: Int32, _ off: Int64, _ whence: Int32
+) -> Int64 {
+  _lseeki64(fd, off, whence)
 }
 
 @inline(__always)