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.
common: Add messages for handling global IO limits

This patch adds LIZ_<MATOCL|CLTOMA>_IOLIMIT* messages that will be used
in the global IO limiting mechanism. It also adds some tools that were
needed to implement these messages, concerning serialization and unit
testing.

Closing #152
common: Fix string and vector<string> seralization

There was a bug in serialization of std::string and
std::vector<std::string>, which (depending on an architecture) may led
to serializing field length as a 64 bit variable, while it was always
deserialized as a 32 bit variable.