Allow --files-from to be used without a destination

[boppreh]

A syntax check currently prevents the option --files-from=list_of_files.txt to be used without a destination (implied --list-only).

$ rsync --files-from=dirs_to_backup.txt --recursive ./
[...]
rsync error: syntax or usage error (code 1) at options.c(2433) [client=3.2.7]

This makes it hard to list all local files from rsync's perspective, that is, after processing of --include, --exclude, --recursive, and IO/permission errors during listing. Note that this list should be independent of destination.

In my case, dirs_to_backup.txt points to directories such as ~/camera/ and ~/.conf/, and I'd like to keep a list of all "files to back up", for validation and troubleshooting purposes. This should be possible even when the remote host is offline. My current workaround is to dry-run a transfer to a temp dir:

$ rsync --files-from=dirs_to_backup.txt [... other flags ...] --dry-run --itemize-changes ./ "$(mktemp -d)"

But this is obviously not ideal, from a clarity and performance perspective.

I think the check that throws a syntax error when !am_daemon && !am_server && argc == 1 was either added incorrectly, or before the introduction of --list-only. This pull request removes this check, allowing the example above to work.