bug#39708: nfs-service-type broken: rpc.mountd respawns too quickly

[Maxim Cournoyer]

Sorry for the spam, but I also managed to get something slightly
different (doesn't seem to be reproducible easily though):

--8<---------------cut here---------------start------------->8---
scheme@(guix-user)> ,m (shepherd service)
scheme@(shepherd service)> (make-forkexec-constructor
  (list 
"/gnu/store/qyz1bncxlkxv44c03sva2akfsyqzkzh5-nfs-utils-2.4.2/sbin/rpc.mountd" 
"--debug" "all"))
$1 = #<procedure 7f5f7e17e600 at shepherd/service.scm:930:8 args>
scheme@(shepherd service)> ($1)
$2 = 920
scheme@(shepherd service)> error in finalization thread: Bad file descriptor

warning: 'waitpid' -1 failed unexpectedly: No child processes
--8<---------------cut here---------------end--------------->8---