Add postmaster address rewrite back to default config

Seems to be accidentally removed in 0.4.0.

Closes #384.

maddy.conf

@@ -1,4 +1,4 @@
-## Maddy Mail Server - default configuration file (2021-03-07)
+## Maddy Mail Server - default configuration file (2021-08-16)
 # Suitable for small-scale deployments. Uses its own format for local users DB,
 # should be managed via maddyctl utility.
 #
@@ -57,6 +57,9 @@ hostname $(hostname)
 
 table.chain local_rewrites {
  optional_step regexp "(.+)\+(.+)@(.+)" "$1@$3"
+ optional_step static {
+ entry postmaster postmaster@$(primary_domain)
+ }
  optional_step file /etc/maddy/aliases
 }