Use anticongestion in security/rkhunter

Introduced in src r316342, the anticongestion feature unifies multiple periodic
scripts' disparate sleeps.

PR: 218446
Submitted by: lukasz@wasikowski.net
Reported by: asomers
Reviewed by: asomers
Approved by: Lukasz Wasikowski <lukasz@wasikowski.net> (maintainer)
Approved by: brd (ports)