Add configuraration option append_dot_mydomain

(from postfix manual..)
append_dot_mydomain (default: yes)

With locally submitted mail, append the string ".$mydomain" to addresses
that have no ".domain" information. With remotely submitted mail, append
the string ".$remote_header_rewrite_domain" instead.

Note 1: this feature is enabled by default. If disabled, users will not
be able to send mail to "user@partialdomainname" but will have to
specify full domain names instead.

Note 2: with Postfix version 2.2, message header address rewriting
happens only when one of the following conditions is true:

 * The message is received with the Postfix sendmail(1) command,
 * The message is received from a network client that matches $local_header_rewrite_clients,
 * The message is received from the network, and the remote_header_rewrite_domain parameter specifies a non-empty value.