BUG/MEDIUM: htx: Catch chunk_memcat() failures when HTX data are formatted to h1

BUG/MEDIUM: htx: Catch chunk_memcat() failures when HTX data are formatted to h1

In functions htx_*_to_h1(), most of time several calls to chunk_memcat() are
chained. The expected size is always compared to available room in the buffer to
be sure the full copy will succeed. But it is a bit risky because it relies on
the fact the function chunk_memcat() evaluates the available room in the buffer
in a same way than htx ones. And, unfortunately, it does not. A bug in
chunk_memcat() will always leave a byte unused in the buffer. So, for instance,
when a chunk is copied in an almost full buffer, the last CRLF may be skipped.

To fix the issue, we now rely on the result of chunk_memcat() only.

This patch must be backported to 2.0 and 1.9.