MINOR: init: do not try to shrink existing RLIMIT_NOFIlE

MINOR: init: do not try to shrink existing RLIMIT_NOFIlE

As seen in issue #1866, some environments will not allow to change the
current FD limit, and actually we don't need to do it, we only do it as
a byproduct of adjusting the limit to the one that fits. Here we're
replacing calls to setrlimit() with calls to raise_rlim_nofile(), which
will avoid making the setrlimit() syscall in case the desired value is
lower than the current process' one.

This depends on previous commit "MINOR: fd: add a new function to only
raise RLIMIT_NOFILE" and may need to be backported to 2.6, possibly
earlier, depending on users' experience in such environments.