archivers/zpaqfranz: Add option for installing dir

bin/dir is just a shorthand for a subcommand of zpaqfranz. Installing it
would conflict with misc/gnuls, so make it optional and disabled by
default.

PR: 265651
Reported by: diizzy (on IRC)
Approved by: Franco Corbelli <franco@francocorbelli.com> (maintainer)
Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D36292