Previously this logic would cap the send to CURL_MAX_WRITE_SIZE bytes,
but for the situations where a larger upload buffer has been set, this
function can benefit from sending more bytes. With default size used,
this does the same as before.
Also changed the storage of the size to an 'unsigned int' as it is not
allowed to be set larger than 2M.
Also added cautions to the man pages about changing buffer sizes in
run-time.
Closes#7022
Some code (e.g. Curl_fillreadbuffer) assumes that this buffer is not
exceedingly tiny and will break if it is. This same check is already
done at run time in the CURLOPT_BUFFERSIZE option.
Replace use of fixed macro BUFSIZE to define the size of the receive
buffer. Reappropriate CURLOPT_BUFFERSIZE to include enlarging receive
buffer size. Upon setting, resize buffer if larger than the current
default size up to a MAX_BUFSIZE (512KB). This can benefit protocols
like SFTP.
Closes#1222
