dotfiles: add some mailx documentation
mail/nailrc: forget ~/.auth

I will use the new 'agent' support later, I think. Or I might change
this into a local-only option. It doesn't work right now anyway (asks
for a password).

(Not that I _use_ mailx anyway, just have it for that 80's feel.)
mail: add stub ~/.mailrc
mail: merge mailrc and nailrc

I don't really use nail/s-nail, but I *really* never use the old Mailx.