Feature Request - List borgfs mounts

`borg mount`

In addition to displaying help and usage for the mount command, `borg mount` can list the currently mounted borg filesystems.

Here is shell command which produces clean output:
`mount | grep borgfs | sed -e 's/borgfs on \(.*\) type.*/\1/'`

output would be:
`/mount/your-mount-point`

Here is a python one-liner which does the same without requiring any new imports to archiver.py:
```
print(re.sub(r".*borgfs (.*) fuse ro,.*", "\\1", "\n".join(s for s in open('/proc/mounts').read().split('\n') if 'borgfs' in s)))
```

If interested I can clean up the one-liner and create a PR. Since this just reads /proc/mounts, this won't work with Windows (not sure about cygwin).

This is easy to test in python shell:
```
import re
print(re.sub(r".*borgfs (.*) fuse ro,.*", "\\1", "\n".join(s for s in open('/proc/mounts').read().split('\n') if 'borgfs' in s)))
```