smb: save the user name in the config even if it matches the current user - fixes #9356 - #9945

What does this change do?

This implements the suggestion in the issue: the default for --smb-user becomes blank, and a blank user is
resolved to the current user name when the remote is used.

The default used to be the name of the user running rclone config. Since rclone doesn't write default values to
the config file, typing your own user name at the prompt left it out of the config, and the remote then logged in
as whoever ran it later - root under a systemd mount. With a blank default, a name typed at the prompt is always
saved, and pressing Enter leaves it blank as before.

Existing configs behave the same: a remote with no user still logs in as the current user, and an explicit user
is unchanged. The ftp backend already resolves a blank user this way.

A blank user = line in the config file was already ignored, so that falls back to the current user as before.
The one difference is an explicitly empty user from somewhere else - --smb-user "", RCLONE_SMB_USER= or
:smb,user='': - which used to send an empty user name and now sends the current user's. The SMB docs already say
anonymous login isn't supported and to use guest instead.

sftp has the same kind of default and could get the same change, but its user handling interacts with the ssh
option, so I've left it for a separate PR if you'd like one.

Testing

• TestNewFsUser: a blank or missing user resolves to the current user, and an explicit one is kept.
• TestUserDefault: the option default is blank.

Both fail on master. I also ran the interactive rclone config from the issue with the new binary: typing the
current user name now writes user = <name>, and pressing Enter leaves it out.

go build ./..., make quicktest and golangci-lint run ./backend/smb/ pass, and
go run ./fstest/test_all -backends smb passes against the docker test servers.

Linked issue

Fixes #9356

Checklist

• This change is trivial OR it has been discussed and agreed in the linked issue.
• I have read the contribution guidelines.
• (If I used AI tools to help write this code) I have read and understood the AI-assisted contributions guidance, and I have tested and take ownership of this change myself.
• I have added tests for all changes in this PR if appropriate.
• I have added documentation for the changes if appropriate. (The option help says a blank user means the current user.)
• All commit messages are in house style.
• (Backend changes only) test_all passes for this backend and if submitting a new backend can provide a test account for the integration tester - see CONTRIBUTING.md.
• This Pull Request is ready for review.