cifs: share lifecycle wiring (REST CRUD -> smb.conf regen + reload; ACL mapping; multiprotocol)

[hartsock]

Goal

Make REST share CRUD actually drive Samba — the analog of the NFS export wiring.

Scope

• protocols/cifs/shares CRUD → regenerate smb.conf + reload smbd
  (idempotent, like the NFS export path; never tear a live share on a no-op).
• Share path resolves under srv_root (the same tree the NFS plane serves) so a
  volume can be exported over both protocols (multiprotocol, ONTAP-faithful).
• ACL mapping (cifs: identity + auth (standalone local users first; AD join deferred) #62) applied per share; share properties
  (oplocks / read only / browsable) mapped to smb.conf.
• protocols/cifs/sessions reads live sessions from Samba.

Dependencies

#61, #62.

Acceptance

• Create a share via REST → smbclient //host/share lists/reads/writes it; delete
  via REST → share gone. ACL changes take effect on reload.