ⓘ Treat Linux active virtual interfaces as secondary IP addresses instead of independent links, fixing alias parsing, hiding invalid status controls, rejecting down-state creation, and removing existing aliases with ip addr del when needed.
Reproduce path:
Example repro before this fix:
1. Go to **Network Configuration → Network Interfaces → Active Now**.
2. Click **Add a new interface**.
3. Enter:
```text
Name: enp0s5:1
IPv4 address: 10.211.55.21
Netmask: 255.255.255.0
Status: Down
```
4. Click **Create**.
Before the fix, Webmin could still create the alias or handle it inconsistently, because `enp0s5:1` is not a real link that can be “down”. It is just an extra IP address on `enp0s5`.
Expected after the fix:
- The UI should not offer `Status` for active virtual aliases.
- If someone submits `up=0` manually anyway, Webmin rejects it with:
`Virtual interfaces cannot be created with down status`
- If an existing active virtual alias is saved as down through lower-level code, Webmin removes the IP using something like:
```bash
ip addr del 10.211.55.21/24 dev enp0s5
```
This PR adds dhcpcd backend support for Debian and Raspberry Pi OS network configuration. It detects dhcpcd only as a final fallback after Netplan, NetworkManager, and ifupdown, preventing Webmin from incorrectly falling back to `/etc/network/interfaces` on dhcpcd-managed systems.
The new backend reads and writes `/etc/dhcpcd.conf`, including DHCP and static IPv4/IPv6 configuration, gateways, static routes, DNS servers, search domains, MTU, and virtual IPv4 aliases. It also supports implicit DHCP-managed interfaces for default dhcpcd setups with no explicit interface blocks, and handles `allowinterfaces` / `denyinterfaces` behavior.
This PR also fixes apply/delete flows for dhcpcd-managed interfaces and virtual aliases, avoids rewriting generated `/etc/resolv.conf`, preserves spacing/comments in touched hosts and nsswitch files, and tightens Active Now handling so virtual aliases are treated as IP addresses rather than independent links.
https://github.com/webmin/webmin/issues/1607
* Note: Preserve existing Netplan indentation when updating DNS settings, validate with netplan generate before applying, and surface apply errors to avoid network loss from malformed YAML.
