[MERGE] enable mode tests for SFTP

[John Arbash Meinel]

In trying to debug some other code, I found a bug which should have been
caught by the permissions tests. In looking closer, I found that they
were not enabled for sftp.

Further, once I enabled them, I found out that 'os.open()' defaults to
creating files with mode 0777, rather than 0666. Which means that all
this time, all of the sftp tests have been creating executable files in
the test directories.

The attached patch re-enables mode tests for sftp, and fixes
StubSFTPServer to use 0666 as the default file mode.

It also refactors SFTPTransport.mkdir() slightly, because I need to use
the helper in another location for another patch I am submitting.

John
=:->

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