MINOR: startup: add set_verbosity

MINOR: startup: add set_verbosity

Let's encapsulate the logic to set verbosity modes (MODE_DEBUG and MODE_VERBOSE)
in a separate function set_verbosity(). This makes the code of main() more
readable and this allows to call set_verbosity() for master process in recovery
mode. So, in this mode, verbosity settings before the master re-execution will
be re-applied to master. set_verbosity() will be extended in future commits to
reduce the verbosiness of master in order not to dump pollers list and filters,
if it was started with -V or -d.