In order to solve the issues with executing 32-bit binaries on
64-bit machines, I'm planning to make the following changes:

1) Add fields to the exec_package structure to specify the
process' address space limits.  These are set by the selected
emulation.

2) Pass these limits to uvmspace_exec() to limit the vm_map
size.

3) Use current limits rather than maximum values when laying
out address space sections during exec.  This mainly affects
the stack segment.

4) Handle resizing process segments inside dosetrlimit().

The following patch solves the first two issues: