Enable mod time syncing on remotes which can't set mod time on an existing object

[ncw]

Currently remotes which can't set the mod time on an existing object are excluded from mod time syncing.

Can fix this by

• Make SetModTime return an error.
  • If the remote isn't capable of setting the mod time on an existing object return a sentinel error.
  • Use this sentinel error to make this OK in the sync routines, and the tests.
  • If an update was required and this sentinel error was returned, then upload the entire file.

This would enable

• b2
• dropbox (with v2 API)

To do modtime syncing.