Added smtp_host_lookup settings to main.cf template

[TheGio]

Tell postfix to use nsswitch if the DNS doesn't work or the needed record is missing

[westbywest]

Given the size of main.cf and the number of parameters already supported, pulling out smtp_host_lookup a parameter with default value "dns, native" seems a better idea. Wanting to identify mail hosts via methods besides DNS, e.g. /etc/hosts entries, can come up when you want to do mail delivery from AWS instances only over their private IP addresses.

[westbywest]

Bump