add: -l USER flag for rsync compatibility

[futpib]

Summary

iroh-ssh works as a RSYNC_RSH (or rsync -e) drop-in for user@host invocations, but rsync sometimes invokes its remote shell as <rsh> -l user host cmd… — and the CLI didn't accept -l, so that path failed at parse time.

This adds -l USER to SshOpts and passes it through to the underlying ssh. With it, RSYNC_RSH=iroh-ssh rsync /local user@<endpoint_id>:/remote works regardless of which form rsync chooses.

The ssh-Command construction was extracted out of start_ssh into a synchronous build_ssh_command helper so it can be unit-tested without spawning a process.

Test plan

• cargo test — new tests in src/ssh.rs:
  • login_user_flag_is_passed_to_ssh — bare flag is forwarded to ssh
  • rsync_invocation_parses_and_builds — full iroh-ssh -l alice <id> rsync --server -e.LsfxCIvu . /tmp/dest invocation parses as Cli and builds an ssh Command with -l alice before the host
• iroh-ssh --help shows the new -l <USER> flag
• Manual: RSYNC_RSH=iroh-ssh rsync … against a real iroh-ssh server

[rustonbsd]

@futpib thanks for the addition. I should do a pass and add all ssh commands in the future ^^, will add that to the todo's.

I tested the rsync locally and I like the testable cmd builder. I will impl #42 next and then release it as v0.2.11 as soon as possible.