Split Debian package into vpp-maglevd + vpp-maglev; add maglevt.1 manpage

vpp-maglevd ships maglevd, maglevd-frontend, both systemd units, and
the config conffiles. vpp-maglev ships maglevc and maglevt as pure
client tools so jump hosts and workstations can install them without
pulling in the daemon. pkg-deb now emits four .debs per release
(2 packages x 2 archs); build-deb.sh takes a package-name argument
and dispatches accordingly.

debian/vpp-maglevd.postinst

@@ -0,0 +1,35 @@
+#!/bin/sh
+set -e
+case "$1" in
+    configure)
+        # Create system user and group if they don't exist.
+        if ! getent group maglevd > /dev/null 2>&1; then
+            addgroup --system --quiet maglevd
+        fi
+        if ! getent passwd maglevd > /dev/null 2>&1; then
+            adduser --system --no-create-home --shell /usr/sbin/nologin \
+                    --ingroup maglevd --quiet maglevd
+        fi
+
+        # Add maglevd to vpp group if it exists (needed for VPP API socket access).
+        if getent group vpp > /dev/null 2>&1; then
+            adduser --quiet maglevd vpp || true
+        fi
+
+        # Upgrade path: older versions of this package shipped the
+        # daemon unit as vpp-maglevd.service. Stop and disable it so
+        # the rename to vpp-maglev.service takes effect cleanly; the
+        # old unit file itself is removed automatically because the
+        # new package no longer owns it.
+        if systemctl list-unit-files vpp-maglevd.service > /dev/null 2>&1; then
+            systemctl stop vpp-maglevd.service || true
+            systemctl disable vpp-maglevd.service || true
+        fi
+
+        systemctl daemon-reload || true
+        systemctl enable vpp-maglev.service || true
+        # vpp-maglev-frontend is intentionally NOT enabled here: the
+        # operator decides whether to expose the web dashboard. Enable
+        # with:  systemctl enable --now vpp-maglev-frontend
+        ;;
+esac