os_mon: cpu_sup should use native sysctl/libkvm calls on BSD

This avoids forking off with os:cmd every time we just want to
collect the load averages. riak does this every second, which
results in a lot of unnecessary load.
os_mon: rename send() to sendi() in cpu_sup.c

Without this, cpu_sup.c won't compile on FreeBSD, as the name
overlaps with the send() socket call.