keyboard: Don't crash if setxkbmap query fails or returns unexpected output

[ashishkr96]

Summary

Fixes #176 — ValueError: too many values to unpack (expected 2) from build_kb_lists() when launching the installer on a live LMDE 7 session.

Root cause

usr/lib/live-installer/main.py:752 parsed the live session's setxkbmap -query output by piping through awk and tuple-unpacking the result of .split() into exactly two names. .split() collapses all whitespace including newlines, so the unpack only succeeds when there are precisely two whitespace-separated tokens in the entire output. Anything else raises ValueError and aborts the installer before the keyboard page can render.

The most likely real-world trigger — and the one consistent with the traceback in #176 — is setxkbmap failing (binary missing on the live image, or an environment where the X-protocol query can't run). When that happens, subprocess.getoutput returns shell error text like sh: 1: setxkbmap: not found — five tokens, instant crash.

Fix

Parse setxkbmap -query line-by-line, picking off model: and layout: if present. If the query fails or the lines aren't there, leave the locals at None. The downstream pre-selection logic at main.py:794-802 already wraps the lookups in except NameError, so an unset/None value just means "no pre-selection" — the user picks from the list manually, which is far better than a hard crash.

Behavior on the happy path is unchanged: normal setxkbmap -query output produces the same (model, layout) pair as before, including the comma-separated form for multi-layout setups.

Test plan

• Python syntax check on the patched file (python3 -m py_compile).
• Unit-tested the new parser against: normal output, multi-layout (us,de), missing setxkbmap (shell error), empty output, partial output (only model: / only layout:), and lines with junk preceding the real ones. Happy path identical to old behavior; all failure modes return (None, None) or (None, value) instead of crashing.
• End-to-end run on a live LMDE 7 ISO on the affected hardware (Dell OptiPlex 755 from Can not install LMDE 7 from usb #176) — left for reviewers with that environment, since the original report could not reach the installer at all without this fix.