Add selective Postfix SRS installation and management

conf/srs.conf.sample

@@ -0,0 +1,22 @@
+# ---
+# - Selective SRS configuration
+# ---
+
+# Public MX hostname used as the MX target of the SRS domain.
+# The installer proposes `hostname -f`; another public FQDN may be selected.
+#
+# Example:
+#
+#    _MX_HOST="mail.example.org"
+#
+_MX_HOST=""
+
+# Domain used as envelope sender domain for SRS rewritten messages.
+#
+# DNS/SPF for this domain must authorize the outgoing mail server.
+#
+# Example:
+#
+#    _SRS_DOMAIN="srs.example.org"
+#
+_SRS_DOMAIN=""