arrange for scp, when in sftp mode, to not ftruncate(3) files early

arrange for scp, when in sftp mode, to not ftruncate(3) files early

previous behavious of unconditionally truncating the destination file
would cause "scp ~/foo localhost:" and "scp localhost:foo ~/" to
delete all the contents of their destination.

spotted by solene@ sthen@, also bz3431; ok dtucker@