delete_bifcs.cgi and save_bifc.cgi only called deactivate_interface()
which brings the link down but never removes the virtual device.
Added destroy_interface_device() that does ip link delete for these.
Also had to add bridge creation (ip link add type bridge) because unlike
bond (81d44f8) and VLAN (7761066) that was still missing.
Other fixes in this commit:
debian-linux-lib.pl: bonding opts were single-element arrays causing
trailing spaces in /etc/network/interfaces on every save.
linux-lib.pl: ip link set up/down was gated behind a
bond/vlan/no-ifconfig check but ip addr add runs unconditionally - so
interfaces got an IP but stayed DOWN. Dropped the unnecessary guard.
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