Handle StartDir differently in case of VirtFS

[lmagyar]

UPDATE: complete rewrite of this comment

Without this PR, pftpd VirtFS behaves errorneously:

• if eg. /storage/emulated/0 is selected as startDir
  • SSHFS fails, can't see anything under /fs
  • WinSCP behaves strangely, it "jumps" from /fs to /fs/storage/emulated/0, that's OK, we can .. out, and when on /fs again, it shows the startDir's content, but can't access the folders in it anymore, so gets broken
• only when / is the startDir, works everything

With this PR:

• the startDir's content is seen under /fs, eg. we have /fs/Android if startDir is /storage/emulated/0 or we have /fs/storage if startDir is /
• we've got real home folders for virtFS SFTP and FTP, pointing to eg. /fs/storage/emulated/0
• both SSHFS and WinSCP works fine (Note: SSHFS must select / as start folder)
• this is analogous to the /saf and /rosaf folders, where we can see the content of the selected SD card folder

---

Just fun fact: I made a version (https://github.com/lmagyar/prim-ftpd/tree/dev-dual-root-and-home-for-sshfs), where depending on SSHFS's start dir, / or nothing=user's home, it was able to see the / folder under /fs (when / is SSHFS's start dir), or the selected startDir, eg. /storage/emulated/0 under /fs (when `` is the SSHFS start dir), it was soo cool, but WinSCP got completely broken and couldn't figure out the reason, even my fake symlink experiments on /fs etc. folders were broken, so gave up. :(

[lmagyar]

Now it's ready to review.

[wolpi]

but WinSCP got completely broken and couldn't figure out the reason

Over the years I had similar issues with some clients 😄

With this PR:

Well, I also see that current code and behavior is not perfect. With current code it would be possible to access files on root folder which is not possible with new code. But would be ok for me to change this.
Wit new code IDE complains about Paths.get() would require a higher API level than the App demands. On my test systems/devices that does not cause an issue but it might for people with old phones.
Could you find a solution without Paths.get() ?

[wolpi]

Oh, and this change breaks my test script.
tests/tests.py
As it expects the old behavior.
Thus some more work is required to switch to new behavior.

[lmagyar]

So, I think I've found finally an even better solution:

• removed all realPath manipulation in VirtualFileSystemView.getFile()
• used the "official" absolute() (SFTP) and getHomeDirectory() (FTP) to create home folders
• works with WinSCP, works with SSHFS
• no tests have to be modified

[wolpi]

That seems to be a good and elegant approach 👍