failtest: don't assume FD_SETSIZE is maximum runtime fd.

failtest: don't assume FD_SETSIZE is maximum runtime fd.

This breaks when rlimit is less.  Unfortunately, valgrind (32 bit x86,
3.7.0.SVN, Ubuntu) fails to set the file limit properly on the test:
reducing it to the obvious getrlimit/setrlimit/getrlimit works fine,
so leaving diagnostics for another day.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
(Imported from CCAN commit a85a809bb17af6b6cf6fa31b300c6622f64ee700)

Autobuild-User: Rusty Russell <rusty@rustcorp.com.au>
Autobuild-Date: Thu Mar  8 06:30:48 CET 2012 on sn-devel-104