[maintainer update] net/headscale 0.18.0

Hi,

update for the latest version 0.18.0 of net/headscale. Replace my
previous post for 0.17.1 version.

Changelog for major version 0.18.0
https://github.com/juanfont/headscale/releases/tag/v0.18.0

Add support for arch i386 and arm64 : headscale v0.18.0 depends on
module modernc.org/libc version v1.21.5 => now supports i386 and arm64
arch (only amd64 in the previous version of this port).

Replace also VARBASE by LOCALSTATEDIR in patch (and modify SUBST_VARS in
Makefile accordingly) following previous comments of sthen@.

Build and tests OK on amd64.
Comments welcome and please commit if OK.

Laurent