ovsdb-dot: Include arrows by default.

ovsdb-dot: Include arrows by default.

When ovsdb-dot generates diagrams for use in the manpages, the dot2pic
postprocessor makes nicer output if the arrowheads are omitted (dot2pic
adds the arrowheads itself).  But for other uses that don't go through
the postprocessor, we generally want the arrowheads.  So this commit adds
an option.  On the principle that the default should be the least
surprising to a naive user, arrowheads are included by default.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: Justin Pettit <jpettit@nicira.com>
Acked-by: Justin Pettit <jpettit@nicira.com>