MINOR: global: add MODE_DISCOVERY flag

MINOR: global: add MODE_DISCOVERY flag

This is the first commit from a series to add a support of discovery mode
in the configuration parser and in initialization sequence.

Discovery mode is the mode, when we read the configuration at the first time
and we parse and set runtime modes: daemon, zero-warning, master-worker. In
this mode we also parse some parameters needed for the master process to start,
in case if we are in the master-worker mode. Like this the master process
doesn't allocate any additional resources, which it doesn't use and it quickly
finishes its initialization and enters to its polling loop. The worker process
after its fork reads the rest of the configuration.

So, let's add in this commit MODE_DISCOVERY flag to check it in
configuration parser functions.