Add support for wireguard peer descriptions

Add support for wireguard peer descriptions

"wgdescr[iption] foo" to label one peer (amongst many) on a wg(4) interface,
"-wgdescr[iption]" or "wgdescr ''" to remove the label, completely analogous
to existing interface discriptions.

Idea/initial diff from Mikolaj Kucharski (OK sthen)
Tests/prodded by Hrvoje Popovski
Tweaks/manual bits from me
Feedback deraadt sthen mvs claudio
OK claudio