devel/kyua: Install kyua.conf as a sample file

Previously, kyua upgrades would override users' changes to
/usr/local/etc/kyua/kyua.conf. Now the upgrade will only affect
/usr/local/etc/kyua/kyua.conf.sample.

PR: 217990
Reported by: ngie
Reviewed by: jmmv, ngie
Approved by: jmmv (maintainer)
Approved by: brd (ports)
Sponsored by: Spectra Logic Corp