Implement new virtual category: net-vpn for VPN related ports

based on discussion at ports@ [1]. As VPN softwares are put in different
physical category net and security. This is a little bit confusing. Let's
give them new virtual category net-vpn.

[1] https://lists.freebsd.org/pipermail/freebsd-ports/2019-April/115915.html

PR: 239395
Submitted by: myself
Approved by: portmgr (mat)
Differential Revision: https://reviews.freebsd.org/D21174