net/tigervnc: separate port into -server -and -viewer

since usually people want to install either server or client on the same
box. Formerly, net/tigervnc had options to build & install vncviewer(client)
but vncviewer could not be installed alone. Server components was necessary.

Now TigerVNC server and viewer are completely separeted and can be installed
independently.

Tested by: Juan Molina
Differential Revision: https://reviews.freebsd.org/D19709