MINOR: limits: prepare to keep limits in one place

MINOR: limits: prepare to keep limits in one place

The code which gets, sets and checks initial and current fd limits and process
related limits (maxconn, maxsock, ulimit-n, fd-hard-limit) is spread around
different functions in haproxy.c and in fd.c. Let's group it together in
dedicated limits.c and limits.h.

This patch is done in order to prepare the moving of limits-related functions
from different places to the new 'limits' compilation unit. It helps to keep
clean the next patch, which will do only the move without any additional
modifications.

Such detailed split is needed in order to be sure not to break accidentally
limits logic and in order to be able to compile each commit separately in case
of git-bisect.