upstream: add a -X option to both scp(1) and sftp(1) to allow

upstream: 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@

OpenBSD-Commit-ID: 27502bffc589776f5da1f31df8cb51abe9a15f1c