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/shares_test.go

@@ -70,12 +70,13 @@ func TestValidation(t *testing.T) {
 			t.Errorf("%q accepted", n)
 		}
 	}
-	for _, p := range []string{"/nas/x", "/tmp_pool"} {
+	for _, p := range []string{"/nas/x", "/tmp_pool", "/mnt/usb", "/srv/media", "/home/alice/docs", "/media/disk", "/variable", "/etcetera"} {
 		if err := ValidatePath(p); err != nil {
 			t.Errorf("%q rejected: %v", p, err)
 		}
 	}
-	for _, p := range []string{"", "/", "nas/x", "/nas/../etc", "/nas/", "/nas/x\"y", "/nas/x\n"} {
+	for _, p := range []string{"", "/", "nas/x", "/nas/../etc", "/nas/", "/nas/x\"y", "/nas/x\n",
+		"/etc", "/etc/samba", "/var/lib/docker", "/usr", "/proc/1", "/root", "/boot/efi", "/run/user/1000"} {
 		if ValidatePath(p) == nil {
 			t.Errorf("%q accepted", p)
 		}
@@ -336,3 +337,27 @@ func TestUsers(t *testing.T) {
 		t.Errorf("owner account deleted")
 	}
 }
+
+func TestParseMounts(t *testing.T) {
+	in := `proc /proc proc rw,nosuid 0 0
+tmpfs /run tmpfs rw 0 0
+/dev/sda2 / ext4 rw 0 0
+/dev/sda1 /boot/efi vfat rw 0 0
+tank /tank zfs rw,xattr 0 0
+tank /tank zfs rw,xattr 0 0
+/dev/sdc1 /mnt/usb\040disk ext4 rw 0 0
+overlay /srv/docker/merged overlay rw 0 0
+`
+	got := strings.Join(ParseMounts(in), "|")
+	if got != "/mnt/usb disk|/tank" {
+		t.Errorf("ParseMounts = %q", got)
+	}
+}
+
+func TestConfigRefusesSystemPath(t *testing.T) {
+	c := Defaults()
+	c.SMB.Shares = []SMBShare{{Name: "etc", Path: "/etc", Guest: true}}
+	if c.Validate() == nil {
+		t.Errorf("share on /etc accepted")
+	}
+}