winhttp: Store credentials set with WinHttpSetCredentials separately from username and password set through options.

qmgr: Implement IBackgroundCopyJobHttpOptions::SetCustomHeaders and IBackgroundCopyJobHttpOptions::GetCustomHeaders.

qmgr: Implement IBackgroundCopyJobHttpOptions::SetSecurityFlags and IBackgroundCopyJobHttpOptions::GetSecurityFlags.

ntdll: Fix the "not found" status set by read_directory_stat() and read_directory_getattrlist() for when restart_scan is set.

Revert "ntdll: Avoid one of the calls to getattrlist() when read_directory_getattrlist() checks the case sensitivity of the file system.".

This was wrong-headed and could never have worked.  The second getattrlist()
call that I was trying to avoid was only necessary when the first fails.  Given
that the first failed, its results can't be used to avoid the second, no matter
what information we requested.

This reverts commit 5514df9d.

This was missed in commit 3494fb7f.