MINOR: cli: "show version" displays the current process version

MINOR: cli: "show version" displays the current process version

This patch implements a simple "show version" command which returns
the version of the current process.

It's available from the master and the worker processes, so it is easy
to check if the master and the workers have the same version.

This is a minor patch that really improve compatibility checks
for scripts.

Could be backported in haproxy version as far as 2.0.