services: nfs: Run rpc.mountd in foreground.

Fixes <https://bugs.gnu.org/39708>.

Shepherd doesn't keep track of processes that fork themselves and would
disable the process after restarting it a couple times.

* gnu/services/nfs.scm (nfs-shepherd-services): Invoke rpc.mountd with the
'--foreground' option.

gnu/services/nfs.scm

@@ -299,6 +299,7 @@ (define (nfs-shepherd-services config)
            (start
             #~(make-forkexec-constructor
                (list #$(file-append nfs-utils "/sbin/rpc.mountd")
+                     "--foreground"
                      #$@(if (member 'mountd debug)
                             '("--debug" "all")
                             '())