common: fix privatefile create.

[0xFFFC0000]

When creating a private file we need to delete the file if exists.

[selsta]

@0xFFFC0000 please also open against release branch

[iamamyth]

This change seems structurally and semantically quite dubious:

1. Create tends to be a one-way operation, very different from drop and recreate. If you want to support a "replace" operation, you'd be better off making a new function which wraps the existing and allow the caller here to optionally request O_EXCL.
2. The existing usage of this function that I see in wallet_rpc_server seems well aware it doesn't remove the existing file; what caller does this help, or contract (documented in the comments or implied by usage) does it honor?

Relevant wallet_rpc_server code:

        rpc_login_file = tools::private_file::create(temp);
        if (!rpc_login_file.handle())
        {
          LOG_ERROR(tr("Failed to create file ") << temp << tr(". Check permissions or remove file"));
          return false;
        }

[0xFFFC0000]

Thanks for your comment @iamamyth
Few points:

1. Apologies for the late reply, I have been extremely busy the past few weeks.
2. I thought about your comment. And your criticism makes sense to me.

In the new push, I am addressing those issues you have raised.

[iamamyth]

Looks good; thanks for spotting and fixing the bug.