add a -X option to both scp(1) and sftp(1) to allow control over

add a -X option to both scp(1) and sftp(1) to allow control over
some SFTP protocol knobs: the copy buffer length and the number of
inflight requests, both of which are used during upload/download.

Previously these could be controlled in sftp(1) using the -b/-R options.
This makes them available in both SFTP protocol clients using the same
option character sequence.

ok dtucker@