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