Add a builder for Agda packages.
Use agda builder for stdlib. mv to all-packages
Merge pull request #3915 from Fuuzetsu/agda-builder

RFC: Agda builder
nixos nat: add description to forwardPorts
Add curl-unix-socket, Run HTTP requests over UNIX socket
Fix ACL2
sudo: add sendmailPath option (defaults to /var/setuid-wrappers/sendmail)

It currently seems impossible to make sudo send emails. No matter how much
debugging I enable, sudo continues to be silent about sendmail. I tried setting
"Defaults mailerpath=/var/setuid-wrappers/sendmail" but that too was completely
ignored (except for logging that the variable mailerpath exists with the value
I assigned to it...).

This enables sendmail support and sets a default value that works on NixOS.