MEDIUM: stats: define an API to register stat modules

MEDIUM: stats: define an API to register stat modules

A stat module can be registered to quickly add new statistics on
haproxy. It must be attached to one of the available stats domain. The
register must be done using INITCALL on STG_REGISTER.

The stat module has a name which should be unique for each new module in
a domain. It also contains a statistics list with their name/desc and a
pointer to a function used to fill the stats from the module counters.

The module also provides the initial counters values used on
automatically allocated counters. The offset for these counters
are stored in the module structure.