docs: Replace Debian blktap-utils package

GRNET ships a different version of blktap-utils enabling
an archipelago block driver, thus the user needs to install
blktap-archipelago-utils.
Bump version to 0.4rc2 after release of 0.4rc1
archipelago: Avoid shell injection
peer: Fix compilation errors

xq_append_head() and xq_pop_head() have a different ABI
after the last libxseg changes.
blktap: Handle various tapdisk states

Until now the paused state was hardcoded as the '0x2a' value
which corresponds only to paused state that hasn't dropped the
logfile. This commit adds all possible tapdisk states and
uses those states to find if the block device is in paused state,
unpaused state or in pausing state.
blktap: Add new class static method

Add new static method for retrieving block device statistics.
Remove 'who' argument from xq functions
mapper: Replace malloc with calloc
mapper: Handle X_UPDATE op
python: Add X_UPDATE op support
python: Fix typo in variable name
mapper: Zero out buffers before writing objects
peer: Fix uninitialized old_pid value
filed: Create all files with RW group permissions
peers: Adjust umask for all peers

Adjust umask for all peers to S_IWOTH, to ensure that every file can have RW
permissions for the group.
archipelago: Adjust permissions of directories

Make all Archipelago directories, files and fifos accessible by the group
Archipelago is configured to run as. This way all Archipelago components can
communicate and signal each other independently from their effective user id.
archipelago: Move cephxid to Radosd class
mapper: Fix mapper log message
archipelago: Fix None value for v0_size
conf: Change the default user/group to archipelago