PID-reuse races fix, introduced GCC validation brakage

Hi,

https://lists.gnu.org/archive/html/dejagnu/2015-07/msg00000.html

this patch introduced  a new failure related to GDB testing, but this
time in GCC guality part of the testsuite.  When
gcc/testsuite/gcc.dg/guality/example.c is executed we have this set of
processes started

PID  PPID     command
100  99          ./example.exe
101  99          cat
102  100        sh -c gdb -nx -nw --quiet > /dev/null 2>&1 ./example.exe
103  102        gdb -nx -nw --quiet ./example.exe

The issue is that when close_wait_program is called pid="100 101" and
when the "wait" returns none of these 4 processes is actually killed,
and the killing of the "kill pipeline" made these pids untouched and
the validation hangs forever.

I not sure what's the best way to fix this issue without
re-introducing the pid race in GDB.  I'm testing a solution which
first gather all the childs processes of the close_wait_program pid
input (with a recursive call of pgrep -P) and then kill them all with
a -15, my assumption is that killing explicitly all the processes
(maybe in the child -> parent order) will avoid the need of stdin
closing and the pid re-use race, but I might be wrong, and all
comments are welcome.  It works so far on a subset of the testsuite,
but I'm still validating this approach.  Now, maybe just the reducing
the sleeping time might workaround the issue....

Cheers,
Yvan