arpd: create /var/lib/arpd on first use

arpd: create /var/lib/arpd on first use

The motivation is to build distributions packages without /var to go
towards stateless systems, see link below (TL;DR: provisionning anything
outside of /usr on boot).

We only try do create the database directory when it's in the default
location, and assume its parent (/var/lib in the usual case) exists.

Links: https://0pointer.net/blog/projects/stateless.html
Signed-off-by: Max Gautier <mg@max.gautier.name>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>