Add comprehensive user management functionality

Features added:
- Create Samba users with password setup (user-create)
- List all Samba users (user-list)
- Delete Samba users with optional system user removal (user-delete)
- Configure share access for users interactively (user-access)
- Revoke user access from specific shares (user-revoke)
- Automatically creates system users if needed
- Interactive share selection for user permissions
- Disables guest access when users are assigned

User management includes:
- Password prompts for new users
- System user creation with no shell access
- Integration with pdbedit for user management
- Automatic valid_users configuration in smb.conf
- Backup creation before any configuration changes

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

README.md

@@ -80,6 +80,39 @@ sudo ./spravuj_sdileni.sh delete <share-name>
 sudo ./spravuj_sdileni.sh auto-disks
 ```
 
+### User Management
+
+**Create a new Samba user**:
+```bash
+sudo ./spravuj_sdileni.sh user-create [username]
+```
+This will:
+- Create a system user if it doesn't exist (with no shell access)
+- Prompt for a Samba password
+- Optionally configure which shares the user can access
+
+**List all Samba users**:
+```bash
+sudo ./spravuj_sdileni.sh user-list
+```
+
+**Configure share access for a user**:
+```bash
+sudo ./spravuj_sdileni.sh user-access [username]
+```
+Interactively select which shares the user can access.
+
+**Revoke user access from a share**:
+```bash
+sudo ./spravuj_sdileni.sh user-revoke <username> <share-name>
+```
+
+**Delete a Samba user**:
+```bash
+sudo ./spravuj_sdileni.sh user-delete [username]
+```
+Optionally removes the system user as well.
+
 ## Share Types
 
 The script creates different types of shares: