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.postrm

@@ -0,0 +1,12 @@
+#!/bin/sh
+set -e
+case "$1" in
+    purge)
+        if getent passwd maglevd > /dev/null 2>&1; then
+            deluser --quiet --system maglevd || true
+        fi
+        if getent group maglevd > /dev/null 2>&1; then
+            delgroup --quiet --system maglevd || true
+        fi
+        ;;
+esac