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.prerm

@@ -0,0 +1,10 @@
+#!/bin/sh
+set -e
+case "$1" in
+    remove|purge)
+        systemctl stop vpp-maglev.service || true
+        systemctl disable vpp-maglev.service || true
+        systemctl stop vpp-maglev-frontend.service || true
+        systemctl disable vpp-maglev-frontend.service || true
+        ;;
+esac