Close both internal pipe fds after VG_(fork) in parent and child

Close both internal pipe fds after VG_(fork) in parent and child

An VG_fork() creates a pipe between parent and child to syncronize the
two processes. The parent wants to register the child pid before the
child can run. This is done in register_sigchld_ignore.

Make sure both the parent and the child close both the read and write
file descriptors so none leak.

https://bugs.kde.org/show_bug.cgi?id=479661