Add dmsystem service for ICMP echo response

[JohnAmadis]

Summary

dmicmp is a Library-type DMOD module - it has no main(), so nothing
spawns it as a process on its own. It only starts answering ICMP Echo
Requests (ping) once something calls dmod_init() by loading it, which
today requires a manual step. This adds a
dmsystem/libsystemd
service so ping response can be turned on (and off) declaratively:

• configs/icmp.ini - a libsystemd unit (type=oneshot, exec=dmell)
  that loads dmicmp at boot.
• configs/icmp-start.dme - the dmell script it runs: module load dmicmp.
• configs/icmp-stop.dme - companion script, module unload dmicmp, to
  stop answering pings. Not wired as an "ExecStop" - libsystemd has no
  such key yet - so it's run directly instead.
• docs/service.md - full rationale: why a Library module needs a script
  instead of exec=dmicmp directly, why type=oneshot, and why there's
  no service stop icmp.
• dmicmp.dmr gets a configs= entry so configs/ is installed
  alongside the module (mirrors the dmtty/tools/console convention for
  shipping a unit file next to its module).
• README/docs index updates pointing at the new service.

This establishes the first .dme-as-exec= unit in the DMOD ecosystem
(researched across the other module repos - nothing else uses this
pattern yet, though dmtty/tools/console established the
configs/<unit>.ini + .dmr install convention this mirrors).

Test plan

• No C/CMake changes - configs//docs/ are data files only, build
  unaffected.
• Manual: install dmicmp, drop configs/icmp.ini into a libsystemd
  units directory (with args= pointed at the installed
  icmp-start.dme), boot systemd, confirm the device answers ping;
  run icmp-stop.dme and confirm it stops.

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https://claude.ai/code/session_01XcEbAA3VGWGpAoM2pMeTkv

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