This is in preparation of logging to syslog.
If an empty directory is overwritten we will always get ENOTEMPTY as the "empty" directory will still contain gocryptfs.diriv. Handle that case by removing the target directory and trying again. Fixes issue #10
...and minimal comment changes.
This patch also splits off Mkdir and Rmdir into its own file. Fixes issue #8, thanks to @diseq for the bug report.