Make sure elevated mount-point evaluates to a single line in GNU/Linux (#488)
Make sure elevated mount-point evaluates to a single line in GNU/Linux (#488)

Consider the case where a previously elevated mount was not unmounted
correctly. `mount | grep $(basename $APPIMAGE)` would evaluate to a
multi-line string, and therefore `awk` will perform its work on *each*
line separately, usually returning something like:

```
/tmp/.mountXXXX
/tmp/.mountYYYY-elevated
```

We then pass `$mountpoint` to `mkdir -p`, which evaluates to:

```
mkdir -p /tmp/.mountXXXX
/tmp/.mountYYYY-elevated
```

Therefore `/tmp/.mountXXXX` gets created, and
`/tmp/.mountYYYY-elevated` is ran as an executable, causing all sort of
cryptic errors to come out. This can be even worse, since the third
column of a `mount` line is not always a path, so the errors make even
less sense.

Fixes: https://github.com/resin-io/etcher/issues/482
Fixes: https://github.com/resin-io/etcher/issues/459

Signed-off-by: Juan Cruz Viotti <8493db2a4f45d66df375afe12f7eba59fe1c8d78@gmail.com>
cedricve/etcher,resin-io/herostratus,resin-io/etcher,resin-io/etcher,resin-io/resin-etcher,resin-io/etcher,resin-io/etcher,cedricve/etcher,resin-io/herostratus,cedricve/etcher,resin-io/etcher,resin-io/resin-etcher