Keep the reading side of the FIFO opened after a vgdb disconnect
Otherwise, if the valgrind process has just forked before
the vgdb connect, and the child has not yet closed the FIFO it inheritates
from its parent, and a 2nd vgdb is started before the parent has
re-opened the reading side FIFO, the 2nd vgdb can connect to the child,
which will not handle the wakeup character/connection request, and
vgdb will have a write error and/or block.



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13944 a5019735-40e9-0310-863c-91ae7b9d1cf9