Use a single sftp_senddata() to send each SFTP packet, rather than

Use a single sftp_senddata() to send each SFTP packet, rather than
using one for the length field and one for the rest of the packet
contents. Since sftp_senddata() has no queuing or deferral mechanism
but instead constructs and sends an SSH2_MSG_CHANNEL_DATA message
immediately, this change has the effect of ceasing to split every SFTP
packet across two SSH messages.

[originally from svn r9603]