MINOR: config: round up global.tune.bufsize to the next multiple of 2 void*

MINOR: config: round up global.tune.bufsize to the next multiple of 2 void*

Since HTX casts the buffer to a struct and stores relative pointers at the
end, it is mandatory that its end is properly aligned. This patch enforces
a buffer size rounding up to the next multiple of two void*, thus 8 on
32-bit and 16 on 64-bit, to match what malloc() already does on the beginning
of the buffer. In pratice it will never be really noticeable since default
sizes already are such multiples.