Fix Windows SFTP open flags and wolfsshd -D argument parsing(f8827) - #1173

Summary

• wolfSSH_SFTP_RecvOpen()'s Windows (USE_WINDOWS_API) path built dwCreationDisposition by OR-ing together OPEN_EXISTING / CREATE_ALWAYS bits, but CreateFile()'s creation-disposition parameter is a single enumerated value, not a bitmask. TRUNC and EXCL were also never wired up (left under #if 0), and APPEND access was missing. Add SFTP_WinCreationDisp() to resolve the SFTP CREAT/EXCL/TRUNC flag combination to the one correct CreateFile() disposition, and OR in FILE_APPEND_DATA for WOLFSSH_FXF_APPEND.
• wolfsshd's -D (foreground/no-daemon) argument check on Windows compared cmdArgs[i] with WSTRCMP, but cmdArgs entries come from CommandLineToArgvW and are wide strings. Comparing them as narrow char* data meant -D was never recognized. Use wcscmp() against L"-D" instead.
• Extend the WOLFSSH_TEST_INTERNAL regression-test plumbing in wolfsftp.c/wolfsftp.h to build under USE_WINDOWS_API too (it was previously guarded out on Windows), except for wolfSSH_SFTP_TestInvalidateHeadFd(), which stays POSIX-only since it manipulates a raw fd and Windows tracks a HANDLE instead.
• Add TestSftpWindowsOpenFlagMatrix() (tests/regress.c), which walks the RecvOpen CREAT/EXCL/TRUNC flag matrix on Windows and checks both the open result and the resulting file state for each case:
  • WRITE only, no CREAT, missing file -> fails, file not created
  • WRITE|CREAT, missing file -> creates it (OPEN_ALWAYS)
  • WRITE|CREAT, existing file -> opens without truncating
  • WRITE|CREAT|TRUNC, existing file -> truncates immediately
  • WRITE|CREAT|EXCL, existing file -> fails
  • WRITE|CREAT|EXCL, missing file -> succeeds
  • READ|WRITE|CREAT, missing file -> creates it

Testing

• Built and ran the full test suite on Windows via MSYS2 MinGW64 (_WIN32 -> USE_WINDOWS_API):

  PASS: tests/api.test.exe
  PASS: tests/testsuite.test.exe
  PASS: tests/kex.test.exe
  PASS: tests/regress.test.exe
  PASS: tests/unit.test.exe
  

  tests/regress.test.exe includes the new TestSftpWindowsOpenFlagMatrix(), exercising the corrected CreateFile() disposition logic end to end.

• scripts/external.test and scripts/fwd.test are skipped on Windows as expected (external network / Unix-only port forwarding).