cli: don't cast away const in select() timeout argument

The timeout argument to select() is modified to reflect the time
remaining when the function returns on a non-timeout condition.
Passing a pointer to const data and casting away the const-ness is
asking for trouble, but for some reason, this trouble manifests
itself only on non-x86 architectures [whose implementation of select()
in glibc is different from the one supplied for x86]

Fix this by passing a stack copy of the timeout argument to select()