security/unix-selfauth-helper: Add new port

[zirias]

Summary

This little helper enables self-authentication against the local passwd
database using pam_exec.

[zirias]

This depends on the fix in https://reviews.freebsd.org/D35169

When reviewing, please consider having a look at the actual code here: https://github.com/Zirias/unix-selfauth-helper/tree/v1.0

[tcberner]

Moin moin

Tested against the upcoming KDE Plasma 5.25 desktop that will also required this feature.

I would suggest that you include a sample pam-File that other ports can use as a template (well, it is in the man already, but...)

mfg Tobias

[zirias]

Add example pam service policy configuration

Ok, added an example for those who don't read the manpage ;) But it's probably
a good place to add some instructions for port maintainers.

While here, also patch the manpage to always have the correct LOCALBASE.

[zirias]

Remove example file, improve manpage instead.

This latest change made the port unnecessarily complex. Instead rerolled v1.0
tag to include all the hints and infos directly in the manpage.

[zirias]

Fix building as non-root.