x11/xscreensaver: update to 6.02

Only install xscreensaver suid root if option PAM is disabled. In this
case, add an extra patch, so xscreensaver-gfx drops privileges
(otherwise it doesn't show any screensaver).

Fix setuid installs in the Makefile and remove extra patch. Respect
upstream's USE_CSTD=gnu89. Remove now-unknown configure option and
associated dependency. Remove old CONFLICTS.

Use security/pam_helper for PAM support. This is the last release
to support pam_unix as the next release removes external helper
calling code.

PR: 254178
Co-authored-by: swills, vishwin
Approved by: arrowd (mentor), maintainer-timeout, maintainer-reset