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/mounts.go

@@ -0,0 +1,55 @@
+package shares
+
+import (
+	"sort"
+	"strconv"
+	"strings"
+)
+
+// pseudoFS are filesystem types that never hold user data.
+var pseudoFS = map[string]bool{
+	"proc": true, "sysfs": true, "tmpfs": true, "devtmpfs": true, "devpts": true, "cgroup": true, "cgroup2": true,
+	"securityfs": true, "pstore": true, "bpf": true, "debugfs": true, "tracefs": true, "configfs": true,
+	"fusectl": true, "mqueue": true, "hugetlbfs": true, "autofs": true, "binfmt_misc": true, "nsfs": true,
+	"overlay": true, "squashfs": true, "efivarfs": true, "rpc_pipefs": true, "nfsd": true, "ramfs": true,
+}
+
+// ParseMounts extracts shareable mountpoints from /proc/self/mounts content:
+// real filesystems outside system directories, sorted and de-duplicated.
+func ParseMounts(data string) []string {
+	seen := map[string]bool{}
+	var out []string
+	for _, line := range strings.Split(data, "\n") {
+		f := strings.Fields(line)
+		if len(f) < 3 || pseudoFS[f[2]] {
+			continue
+		}
+		mp := unescapeMount(f[1])
+		if !strings.HasPrefix(mp, "/") || ValidateNotSystem(mp) != nil || seen[mp] {
+			continue
+		}
+		seen[mp] = true
+		out = append(out, mp)
+	}
+	sort.Strings(out)
+	return out
+}
+
+// unescapeMount decodes the octal escapes (\040 etc.) the kernel uses in mounts.
+func unescapeMount(s string) string {
+	if !strings.Contains(s, `\`) {
+		return s
+	}
+	var b strings.Builder
+	for i := 0; i < len(s); i++ {
+		if s[i] == '\\' && i+4 <= len(s) {
+			if n, err := strconv.ParseUint(s[i+1:i+4], 8, 8); err == nil {
+				b.WriteByte(byte(n))
+				i += 3
+				continue
+			}
+		}
+		b.WriteByte(s[i])
+	}
+	return b.String()
+}