REORG: startup: move nofile limit checks in limits.c

REORG: startup: move nofile limit checks in limits.c

Let's encapsulate the code, which checks the applied nofile limit into
a separate helper check_nofile_lim_and_prealloc_fd(). Let's keep in this new
function scope the block, which tries to create a copy of FD with the highest
number, if prealloc-fd is set in the configuration.