BUG/MINOR: haterm: preserve the pipe size margin for splicing

BUG/MINOR: haterm: preserve the pipe size margin for splicing

Originally in httpterm we used to allocate 5/4 of the size of a pipe to
permit to use vmsplice because there's some fragmentation or overhead
internally that requires to use a bit of margin. While this was initially
applied to haterm as well, it was accidentally lost with commit fb82dece47
("BUG/MEDIUM: haterm: Properly initialize the splicing support for haterm"),
resulting in errors about vmsplice() whenever tune.pipesize is set. Let's
enforce the ratio again.

No backport is needed.