- no need to initalise mutexes, their zero value is valid
- make NewServer simpler, 90% of use cases already had an
io.ReadWriteCloser, in the single case that it it is easy to provide a
simple wrapper.
Replaced panic with log.Fatal
Removed old commented code line
Cleaned up declaration of flag variables
Fixed rootDir flag variable - was pointing to debugLevelStr
Log any final error returned by SFTP subsystem