mail/filtermail: update 0.8.3 -> 0.8.6

• fix build with more recent flex versions
• pet portlint

PR: 247771
Submitted by: Mike Clarke <jmc-freebsd2@milibyte.co.uk> (maintainer)
MFH: 2020Q3
Relnotes: https://raw.githubusercontent.com/nondeterministic/mailfilter/master/NEWS