x11/xscreensaver: Update to 6.04

Any code for calling an external helper for PAM was removed in 6.03,
therefore replace pam_helper by unix-selfauth-helper that's called by
pam_exec.so

Also fix line length in USES.

Approved by: x11 (manu), tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D35348