misc: Add systemd service file to release packages

According to [1] "/usr/lib/systemd/system/" is where systemd files from installed packages are stored in Arch Linux

Big thanks to @jordansissel for the in-depth explanation of this PRs effects:
> This PR replaces --prefix with the fpm dir package type's feature "path mapping" which lets you map a local file to a destination file when creating the package using this syntax: localpath=destinationpath

[1] https://wiki.archlinux.org/title/Systemd#Writing_unit_files

misc/fpm-pack.sh

@@ -119,5 +119,5 @@ fpm \
 	--package "${DIR}/${VERSION}/${DISTRO}/${OUTPUT}" \
 	${CHANGELOG} \
 	${DEPS} \
-	--prefix "$PREFIX" \
+	"misc/mgmt.service"="/usr/lib/systemd/system/mgmt.service" \
-	"$BINARY"
+	"$BINARY"="$PREFIX/mgmt"