util: Try to get limits from /proc

util: Try to get limits from /proc

Calling prlimit() requires elevated privileges, specifically
CAP_SYS_RESOURCE, and getrlimit() only works for the current
process which is too limiting for our needs; /proc/$pid/limits,
on the other hand, can be read by any process, so implement
parsing that file as a fallback for when prlimit() fails.

This is useful in containerized environments.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>