librumphijack reserves for rump to use.
This is not normally a problem, as most applications don't attempt
to use very high fds - but /bin/sh does.
This fix is something of a kludge - really the apparent fd resource limit
ought to be lowered as well, but this is sufficient to allow the shell
to work (when its dup2() gets rejected, it just tries again with a smaller
target fd until it eventually succeeds.) This fixes the librumphijack
shell ATF tests.
A better, more comprehensive, fix would be good...
1ec349e914