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/web/web_test.go

@@ -89,7 +89,18 @@ func newTestServer(t *testing.T) (*Server, *sched.Scheduler, *store.Store) {
 		t.Fatal(err)
 	}
 	srv := New(cfg, st, ring, sc, shs)
-	srv.dirExists = func(p string) bool { return strings.HasPrefix(p, "/tank") || strings.HasPrefix(p, "/tmp_pool") }
+	srv.dirExists = func(p string) bool {
+		return strings.HasPrefix(p, "/tank") || strings.HasPrefix(p, "/tmp_pool") || strings.HasPrefix(p, "/srv")
+	}
+	srv.realPath = func(p string) (string, error) {
+		if p == "/srv/evil" { // symlink into a system directory
+			return "/etc", nil
+		}
+		return p, nil
+	}
+	srv.readMounts = func() string {
+		return "proc /proc proc rw 0 0\n/dev/sda2 / ext4 rw 0 0\ntank/data /tank/data zfs rw 0 0\n/dev/sdc1 /mnt/usb ext4 rw 0 0\n"
+	}
 	return srv, sc, st
 }