Derive the DHCP subnet from the FOG address, not the interface (GH-1747)

[mastacontrola]

Problem

On an interface with more than one IPv4 address, the installer writes a DHCP subnet that is not a network. #1747 shows a saved mask of 255.255.0.0 on a /24, a Kea subnet of "10.0.45.2/24", a pool starting at .12, and arithmetic syntax error ... /8 during interface selection.

This is the working-1.6 side of #1753, which fixed dev-branch.

Cause

Each helper reads "the interface" and takes a different address from it:

Where	What it took
input.sh	every inet address, link-local 169.254.x.x included, so a link-local address listed first became the primary
getCidr()	the second global address's prefix (head -n2 | tail -n1), through an unanchored grep, so eth1 also read eth10
writeKeaSample	every global address, unquoted, into mask2network, so a second address became the mask
configureDHCP	the first global address, which is not always ${NET_fog_server_ip}
interface2broadcast()	the first brd, whichever address it belonged to

A second address in the same /24 gives exactly the reported config. A global 169.254.x.x/16 next to the /24 gives the saved 255.255.0.0. A link-local-only NIC gives the /8 error.

Fix

Everything derives from ${NET_fog_server_ip}, the address FOG advertises as next-server.

• input.sh takes global addresses only, and never 169.254.0.0/16.
• getCidr IFACE [ADDR] and interface2broadcast IFACE [ADDR] read the named address on that interface only. Both stay awk, so they work with busybox (Installer support for Alpine Linux as host OS #863).
• configureDHCP and writeKeaSample compute the network from ${NET_fog_server_ip}.
• cidr2mask with no prefix returns nothing, instead of printing the arithmetic error.
• mask2cidr sends its error message to stderr instead of into the caller's value.
• The ifconfig fallbacks in input.sh and configureDHCP are removed. ifconfig is not installed before the package step, and the input.sh one stored a prefix length as the mask.

Behavior changes

• With -y, an interface with no usable address now exits. It used to repeat the same answer forever.
• Default storage group trust. storageDefaultCidr now takes the prefix of ${NET_fog_server_ip}. On a multi-address interface it used to take the second address's prefix and apply it to the FOG address. On an interface with one address, nothing changes. This is the trusted CIDR for node-to-node status calls, so this PR is a draft until that change is approved.

Test

tests/installer-subnet-detection.test.sh replays address layouts through the real functions and the real input.sh lines. The fake ip prints real ip -o output.

• On the old code: 21 passed, 37 failed. It reproduces the report exactly.
• After the fix: 58 passed, 0 failed.
• Alpine: the helpers ran against busybox ip and awk on a dummy interface with the same addresses, with the same results.

Refs #1747

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