REORG: startup: move code that applies limits to limits.c

REORG: startup: move code that applies limits to limits.c

In step_init_3() we try to apply provided or calculated earlier haproxy
maxsock and memmax limits.

Let's encapsulate these code blocks in dedicated functions:
apply_nofile_limit() and apply_memory_limit() and let's move them into
limits.c. Limits.c gathers now all the logic for calculating and setting
system limits in dependency of the provided configuration.