Certain getrlimit(2) resource limits are meant to limit resource
consumption on a per-UID basis and use the 'uidinfo' structure
to track the total resource consumption for a given UID.
However, the limits themselves are still stored in 'struct proc'
which makes it impossible to propagate a newly modified limit value to
all processes owned by the UID whose limit we wish to change.
This change addresses this issue by adding a new 'struct plimit'
member to 'struct uidinfo' and uses it to store resource limit values
for per-UID limits. The 'lim_rlimit' function will now distinguish
between per-process and per-UID limits and return the limit value
from the appropriate location.