Use filesystem name mapping on FreeBSD

[am11]

FreeBSD debug build was running into:

$ ~/.dotnet9/dotnet new console -n app1
...
Process terminated. Assertion failed.
GetFileSystemType returned 222
   at Interop.Sys.TryGetFileSystemType(SafeFileHandle fd, UnixFileSystemTypes& fileSystemType)
   at Microsoft.Win32.SafeHandles.SafeFileHandle.Init(String path, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, Int64& fileLength, UnixFileMode& filePermissions)
   at Microsoft.Win32.SafeHandles.SafeFileHandle.Open(String fullPath, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, UnixFileMode openPermissions, Int64& fileLength, UnixFileMode& filePermissions, Boolean failForSymlink, Boolean& wasSymlink, Func`4 createOpenException)
   at System.IO.Strategies.OSFileStreamStrategy..ctor(String path, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, Nullable`1 unixCreateMode)
   at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize)
   at NuGet.Protocol.RemotePackageArchiveDownloader.CopyNupkgFileToAsync(String destinationFilePath, CancellationToken cancellationToken)
   at System.Runtime.CompilerServices.AsyncMethodBuilderCore.Start[TStateMachine](TStateMachine& stateMachine)
   at NuGet.Protocol.RemotePackageArchiveDownloader.CopyNupkgFileToAsync(String destinationFilePath, CancellationToken cancellationToken)
   at NuGet.Packaging.PackageExtractor.<>c__DisplayClass5_0.<<InstallFromSourceAsync>b__0>d.MoveNext()
   at System.Runtime.CompilerServices.AsyncMethodBuilderCore.Start[TStateMachine](TStateMachine& stateMachine)
....

Turned out that 222 (aka 0xDE) is some kind of a partial magic number (e.g. https://people.redhat.com/bhubbard/cov-html-int-2019-10-09/1/574compat.cc.html uses 0xDE, while https://github.com/coreutils/coreutils/blob/83ec7a706a9f931e418ec6a084587b8995ca5388/src/stat.c#L556 is using 0x2FC12FC1 which is the same we have.

I think it's best to use name mapping rather than the alternative to use some fake name like zfs1 which f_fstypename will never return on other systems.

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[adamsitnik]

LGTM, thank you @am11 !