The `credit_` prefix may seem redundant within the REIIDEN.CFG
structure, but consistency seems more important here. Makes it much
easier to follow how these fields are copied around.
Part of P0229, funded by Ember2528.
Turns out I've misread the execl() documentation. That function does in
fact *not* need a second `nullptr` parameter for envp, because it
doesn't pass envp.
Part of P0216, funded by JonathKane.