Shares: allow any folder, not only ZFS datasets

SMB shares and NFS exports may now point to any existing directory
(USB disks under /mnt or /media, /srv, home folders...) instead of only
ZFS dataset mountpoints. System trees (/etc, /usr, /var, /boot, /proc,
/root, ...) are refused, also after resolving symlinks. The path
suggestions list mounted data filesystems from /proc/self/mounts in
addition to ZFS mountpoints.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/shares/render.go

@@ -102,7 +102,7 @@ func RenderExports(c Config, uid, gid string) string {
 		default:
 			opts = append(opts, "root_squash", "anonuid="+uid, "anongid="+gid)
 		}
-		// crossmnt: child ZFS datasets are separate mounts and must be reachable
+		// crossmnt: child mounts (ZFS datasets or other filesystems) must be reachable
 		opts = append(opts, "crossmnt", "no_subtree_check")
 		var parts []string
 		for _, cl := range e.Clients {