Follow up to rev 13944

Follow up to rev  13944
13944 objective was to avoid having a vgdb that connects to a just forked child
that would have the FIFO still opened, while its parent would close it.

However, in case a previous vgdb closed the FIFO, the read FIFO in the parent
is put in 'eof status' by the kernel. So, readchar will then return eof
in the parent unless another vgdb re-opens the FIFO in write.
So, gdbsrv does not stop anymore on error if needed, due to this readchar
giving eof.

The only way to reset this eof condition is to close the fd.
But we must always have the FIFO open (to avoid the race condition that
rev 13944 fixed)
=> in case of error, first re-open the FIFO, before closing the (previous)
FIFO fd (which is in eof state and cannot be properly used anymore).

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14134