Feature: postUnmount plugin callback

[buhtz]

That issue is a followup of the un-merged and stale PullRequest #1269 .

The PR was open for two years. No one worked further on it. To get it out of the pipeline it is transformed into this issue. Please feel free to reopen the PR and work further on it.

Original PR text

This is useful for unmounting the remote drive over ssh after sshfs is unmounted.

If the remote drive is unmounted before sshfs with the existing callback, then sshfs can not be unmounted and the mountpoint shows up as

d????????? ? ?         ?            ?            ? mountpoint

until the remote drive is remounted.

The existing unmount() function was not renamed to preUnmount() in order to not break any existing plugins.