net-p2p/bitcoin: update to 0.16.3

High priority update, see:

• https://bitcoin.org/en/alert/2018-09-21-required-upgrade
• https://bitcoincore.org/en/2018/09/20/notice/

Approved by: timur (mentor)
Security: CVE-2018-17144
Differential Revision: https://reviews.freebsd.org/D17345