timeval: Add monotonic time functionality for NetBSD and FreeBSD.

This patch also checks the system platform as clock_gettime
could exist on different platforms but with different values of
CLOCK_MONOTONIC and different definitions of 'struct timespec'.
In this case, the system call would be expected to catch the
error, which is dangerous.

This patch ensures Linux, NetBSD and FreeBSD platforms use
clock_gettime with their corresponding correct values and
definitions. All other platforms use time.time().

Signed-off-by: Ryan Wilson <wryan@nicira.com>
Acked-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>