security/libkpass: Update to version 6

• Change from Sourceforge to GitHub for upstream, as per note on SF page
• Add LICENSE_FILE to demonstrate GPLv3 license
• Delete patch-src_kpass.c as upstream incorporates similar changes

Changes this release:

https://github.com/bldewolf/libkpass/blob/6/ChangeLog

Approved by: vg (maintainer timeout), mat (mentor)
Differential Revision: https://reviews.freebsd.org/D8034