The IPv4 address and gateway were required even for DHCP or no IPv4, so
such an interface could not be created; malformed addresses passed and a
missing prefix crashed the XML builder. They are validated like the IPv6
fields now (netmask notation accepted, IPv6 scope ids rejected), and a
static address without a gateway gets no empty route. Interface names
follow the Linux rule (up to 15 letters, digits, . _ -). A bridge member,
or the device an ethernet interface configures, must be a device of the
host (the page no longer falls back to made-up names), a bridge cannot
contain itself, and needs STP and a non-negative delay.
The form checked both with re.match("^[0-9a-f./:]+|^$"), which tests
only how a value starts: "2001:db8::10/64'/><evil/>" passed and went
into the interface XML, which is built with f-strings, while upper-case
hex was refused. An address without a prefix passed too and then failed
with a ValueError when the XML was built.
The address is parsed with ipaddress.IPv6Interface and stored as
address/prefix (/128 when none is given), the gateway with
IPv6Address. A static configuration needs an address, and the XML gets
a route only when a gateway is given instead of gateway=''. The HTML
patterns accept upper-case hex.
Eight of the ten form and XML checks fail on the previous code.
