After authentication is complete, unveil login.conf "r" (to discover

After authentication is complete, unveil login.conf "r" (to discover
login tweaks), and walk the path (would could be $PATH, or not) adding
an unveil "x" to each possible executable which execvpe() could later
be tried by execvpe.
It would be great if we could do some unveil before authentication, but
I failed to determine all the possibilities.
Some problem reports by bluhm, tb, florian, and others during development
In snaps for about 3 weeks.
ok florian