When relocating preserved fds, start with the highest ones first

When relocating preserved fds, start with the highest ones first
to avoid moving fds around more than we have to.  Now uses a bitmap
to keep track of which fds are being preserved.  Fixes a bug where
the debugging fd could be relocated to the same fd as the error
backchannel temporarily, resulting in debugging output being printed
to the backchannel if util@debug was enabled.

--HG--
branch : 1.8