prlimit: fix case when PID is given later

prlimit: fix case when PID is given later

 # prlimit --nofile=:4000 --pid $$

When the PID is given later than a partially given limit, then prlimit
used the current PID for getting the missing part (hard, soft) of the
limit.

Factored out the retrieval of the unknown limit from parse_prlim() to
new get_unknown_hardsoft() which is to be called by do_prlimit() based
on the struct prlimit member 'modify' set by add_prlim().

[kzak@redhat.com: - use prlimit->mofify as mask
                  - add soft vs. hard limit check from another
                    Bernhard's patch]

Signed-off-by: Bernhard Voelker <mail@bernhard-voelker.de>
Signed-off-by: Karel Zak <kzak@redhat.com>