mount: Separate fuse logic from the filesystem logic

Separate fuse logic from the filesystem logic itself, in order to be able
to replace fuse interface with some other one.

Change-Id: I6cd6c08d00d7516803a9c79924fe4272ee6be32c
tests: Generate GIDs different from UIDs in setup_machine.sh

This commit modifies setup_machine.sh to create users with UID different
from their primary GID, so that we can spot bugs where someone uses gid
instead of uid or otherwise in the code. The change is backward
compatible, so re-running this script will change existing GIDs.

Change-Id: Iafd235565809bb97c7e8f4f30c4856312f2a45f1
common, master: Add messages for global I/O limiting status

Global I/O limiting works but there's no possibility to check its current state
- the current configuration. This commit adds network messages for this
operation and prepares the master server to handle these queries.

Change-Id: I584b25757c7bd7af322f8ca15189efaa759206ce
probe: Add iolimits-status command

This commit adds a command 'iolimits-status' which asks indicated master server
for the current global I/O limiting configuration and prints it. The probe is
also added in global I/O limiting reconfiguration test.

Change-Id: I22a8f023006af5dd467b2b926b3d0ace59e6cf98