net/tigervnc: update 1.8.0 -> 1.9.0

While here,

• adjust USE_XORG
• bring IGNORE line earlier (-devel port)
• remove tridiavnc from CONFLICTS since it no longer exists
• some cosmetic changes on Makefile

Approved by: pi (mentor)
Relnotes: https://github.com/TigerVNC/tigervnc/releases/tag/v1.9.0
Differential Revision: https://reviews.freebsd.org/D16340