C99 says that the result should be the number of bytes that would have
been emitted given a large enough buffer, not the number we actually
were able to put in the buffer.  It's time to make our substitute
implementation comply with that.  Not doing so results in inefficiency
in buffer-enlargement cases, and also poses a portability hazard for
third-party code that might expect C99-compliant snprintf behavior
within Postgres.

In passing, remove useless tests for str == NULL; neither C99 nor
predecessor standards ever allowed that except when count == 0,
so I see no reason to expend cycles on making that a non-crash case
for this implementation.  Also, don't waste a byte in pg_vfprintf's
local I/O buffer; this might have performance benefits by allowing
aligned writes during flushbuffer calls.

Discussion: https://postgr.es/m/17245.1534289329@sss.pgh.pa.us