sshkeys: fix missing directory on empty set

[lucab]

This fixes a buggy interaction between the file writing logic and
the set of SSH keys being empty.
Afterburn tries to either write or remove a file, then sync the
parent directory. In both cases, the directory must exist in order
to be successfully sync'ed.

Closes: #317

[bgilbert]

I'm okayish with this, but wouldn't it be cleaner to skip the directory sync if the directory doesn't exist? That would correctly sync in both cases where we do something, and skip the sync only in the case where it doesn't matter anyway.

[jlebon]

I'm okayish with this, but wouldn't it be cleaner to skip the directory sync if the directory doesn't exist?

Or slight variation: always sync, but just handle ENOENT gracefully?

I like not creating the authorized_keys.d dir at all if we don't need to.

[lucab]

Thanks both for the feedback! I've switched this to sync_all only if it manages to open the directory, and to ignore the error on ENOENT.