net/tigervnc-server: Use the same default fp for Xvnc as xorg-server

Minor style fixes:
- Respect ${_MAKE_JOBS} in the post-build target, and
- Remove an extra strip(1) invocation in the post-install target.

Approved by:		meta (maintainer)
Differential Revision:	https://reviews.freebsd.org/D37927