tests: Add possibility to generate a code coverage report

To generate such a report:
* Build sources with CMAKE_BUILD_TYPE=Coverage and install the binaries.
* Run the script: coverage.sh prepare <build-dir> to make the files in
  the build dir accessible by processes which run during tests.
* Run the tests with GENTLY_KILL=1 environmental variable. This will ensure
  that all the binaries dump coverage data on exit.
* Generate a HTML report: coverage.sh generate-html <build-dir> <out-dir>.
master, chunkserver, metalogger: Add isalive command

This commit introduces a new command isalive which can be use to test
if a daemon (mfsmaster, mfsmetalogger of mfschunkserser) if running.
The exit status of this command is as follows:
* 0 if the daemon is running
* 1 if the daemon is not running
* other if an error occured during the check

The exit status 1 is reserved to the isalive command and is used only
to indicate that the daemon is down. No other invocation (including
wrong usage, eg. mfsmaster isaliv) will exit with the status of 1.