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>

packaging/README.md

@@ -29,7 +29,10 @@ nasmon has no authentication: keep port 8090 on the LAN only.
 http://<host>:8090/shares manages Samba shares (guest or per-user, files always
 written as the data owner uid 1000), SMB users (smbpasswd; a login-less Unix
 account is created when needed) and NFS exports (host/IP/CIDR clients, rw/ro,
-squash). Definitions are stored in /etc/nasmon/shares.json; nasmon regenerates
+squash). A share can point to any existing folder (ZFS dataset, USB disk under
+/mnt or /media, /srv…) except system directories (/etc, /usr, /var, /boot…),
+also after resolving symlinks. Definitions are stored in
+/etc/nasmon/shares.json; nasmon regenerates
 /etc/samba/smb.conf (whole file, header "Managed by nasmon") and
 /etc/exports.d/nasmon.exports, validates with testparm, then reloads smbd /
 exportfs -ra. A rejected config is rolled back to the .bak copy. Daemons are