Miscellaneous improvements to nmount plugin (#1547)

Miscellaneous improvements to nmount plugin (#1547)

* plugins/nmount: keep `while` & `do` on one line for consistency

* plugins/nmount: sync only that device, which user wants to unmount

* plugins/nmount: replace all instances of `$dev` with `/dev/$dev`

* plugins/nmount: add `--no-user-interaction` option to `udisksctl`

Otherwise the user will be asked for authentication each time he wants
to unmount, say, HDD, since `udisksctl` will try to power it off.

* plugins/nmount: try to mount only existing block devices

* plugins/nmount: do not invoke `lsblk` immediately after mounting

Sometimes `lsblk` fails to provide mountpoint in such a short time frame.

* plugins/nmount: simplify pipe

* plugins/nmount: keep `echo` arguments in a single pair of quotes

* plugins/nmount: report mountpoint only if mounting was successful