Add support for aliased paths, particularly for Win32-OpenSSH.

[WSLUser]

Create a ssh key using Win32-OpenSSH or import from a Linux host to C:\Users\user.ssh. Create an alias link in WSL(2) in your /home/user/.ssh to point to that Windows directory. Verify you can SSH to a remote Linux host if you've ensured ports 22 and 2222 are allowed in the firewall used on Windows (by default it's Windows Firewall and is an easy Powershell command to add). Now install keychain. Restart your WSL session and see this:

 * keychain 2.8.5 ~ http://www.funtoo.org
 * Starting ssh-agent...
 * Adding 1 ssh key(s): /home/user/.ssh/id_rsa
 * Error: Problem adding; giving up

For awareness, the Linux distribution I'm using in WSL is Debian and the kernel is at 5.10.16.3.

I'd like this issue to be fixed to allow accepting an aliased path instead of only a direct path. It's also possible this can be repro'd purely from Linux.

@danielrobbins since you're the maintainer, please investigate this when you get a chance and comment here so viewers of this issue can know it's being looked at.

[WSLUser]

Today I noticed that it worked when before it wasn't. I'm not sure if that means there was some other env var/path issue that was resolved by Debian or kernel or if a patch was submitted but has no visibility to the users. If a patch was created and merged, I would appreciate having it linked to the issue here, especially I don't know if it's just a fluke and will fail again later.

[mrl5]

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