Allow to avoid NUL-byte management for stringinfos and use in format.c.

Allow to avoid NUL-byte management for stringinfos and use in format.c.

In a lot of the places having appendBinaryStringInfo() maintain a
trailing NUL byte wasn't actually meaningful, e.g. when appending an
integer which can contain 0 in one of its bytes.

Removing this yields some small speedup, but more importantly will be
more consistent when providing faster variants of pq_sendint etc.