BUG/MINOR: haload: fix CPU topology detection by omitting forced "nbthread"

Always writing "nbthread" in the generated global configuration forced a
static thread count, preventing HAProxy from using its automatic CPU
topology detection and correct core binding. This caused severe performance
degradation on large multi-core machines. Fix this by omitting the "nbthread"
directive when -t is not explicitly specified, while ensuring <arg_thrd>
is properly initialized to global.nbthread for internal calculations such as
connection and request rates.

Also ensure <arg_thrd> <= <arg_usr> when -R is specified for internal
calculations.