FIX: update sftp after changes in latest nodejs constants

[wdavidw]

Reference to Node.js constants are broken since Node.js version 6.3.0 after #6534. Here's an illustration of the problem:

# Node v6.2.0
[ `node -e "process.binding('constants').S_IFMT" -p` -eq '61440' ]
# Node v6.3.0
[ `node -e "process.binding('constants').fs.S_IFMT" -p` -eq '61440' ]

The proposed pull request fix it in a backward compatible way.

[mscdex]

I think an easier way might be to just change the constants declaration at the top like:

var constants = require('fs').constants || process.binding('constants');

[wdavidw]

Agreed, only "constants" disturbs me in term of semantic, are you ok with "constants_fs = ..."

[mscdex]

I think keeping the name is fine, it's only ever fs constants (that ssh2-streams uses) anyway so there really should be no confusion.

[wdavidw]

Fine, should be ready now

[wdavidw]

Could you please accept this request if everything is ok with u, we need this fix. Thanks

[mscdex]

Landed in c0b0f76. Thanks!