dot2pic: Use "> $@; mv $@.tmp $@" notation to make this reliably fail

Acked-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
dot2pic: Stop assuming the path of the interpreter

On NetBSD, perl is usually installed as /usr/pkg/bin/perl.

Acked-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>