security/easy-rsa: fix confusion of vars file

...and no longer package it as @sample. It is per-PKI, and easyrsa init-pki
will copy vars.example from the distribution, and create a PKI-local copy
named vars. Should fix grembo@'s bug report [1]

add a new pkg-message file to explain this.

while here, add a convenience hardlink easy-rsa to the easyrsa wrapper,
to have an executable matching the package name.

PR: 264415
Reported by: grembo@ (Michael Gmelin)