util: Introduce ovs_assert macro.

An occasionally significant problem with the standard "assert" macro is
that it writes the failure message to stderr.  In our daemons, stderr is
generally redirected to /dev/null.  It's more useful to write the failure
message to the log, which is what the new ovs_assert macro introduced in
this patch does.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>