BUG/MAJOR: http: sample prefetch code was not properly migrated

When ACLs and samples were converged in 1.5-dev18, function
"acl_prefetch_http" was not properly converted after commit 8ed669b1.
It used to return -1 when contents did not match HTTP traffic, which
was considered as a "true" boolean result by the ACL execution code,
possibly causing crashes due to missing data when checking for HTTP
traffic in TCP rules.

Another issue is that when the function returned zero, it did not
set tje SMP_F_MAY_CHANGE flag, so it could randomly exit on partial
requests before waiting for a complete one.

Last issue is that when it returned 1, it did not set smp->data.uint,
so this last one would retain a random value from a past execution.
This could randomly cause some matches to fail as well.

Thanks to Remo Eichenberger for reporting this issue with a detailed
explanation and configuration.

This bug is 1.5-specific, no backport is needed.