Coqide : allow properly closing communication pipes with coqtop
Coqide : allow properly closing communication pipes with coqtop

 NB: it's important to close coqide's descriptors (ide2top_w and top2ide_r)
 in coqtop. We do this indirectly via [Unix.set_close_on_exec].
 This way, coqide has the only remaining copies of these descriptors,
 and closing them later will have visible effects in coqtop. Cf man 7 pipe
 for more details.

 This should avoid the need for Unix.kill on coqtop clients (at least
 when they aren't inside a long computation).

git-svn-id: 44dc50bdcc0f5d1a212c497508dc9452e0b35e2b@15961 85f007b7-540e-0410-9357-904b9bb8a0f7

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