MINOR: startup: split main: add step_init_3

MINOR: startup: split main: add step_init_3

The first part of main(), just after calling the former init() and before
trying to bind listeners, need to be also encapsulated into a separate
step_init_3() as it is. It contains important blocks to register signals, to
apply memory and nofile limits, etc. The order of these blocks should be also
preserved (especially the signals part).

For the moment step_init_3() must be also executed for all runtime modes.