allow the dns api to set srv records

see https://discourse.mailinabox.email/t/create-srv-record-at-the-dns-server/225

CHANGELOG.md

@@ -12,6 +12,7 @@ Control panel:
 
 * Status checks would fail to load if openssh-sever was not pre-installed, but openssh-server is not required.
 * The local DNS cache is cleared before running the status checks using 'rncd' now rather than restarting 'bind9', which should be faster and wont interrupt other services.
+* The DNS API now allows the setting of SRV records.
 
 Misc:
 

management/dns_update.py

@@ -669,7 +669,7 @@ def set_custom_dns_record(qname, rtype, value, env):
 			v = ipaddress.ip_address(value)
 			if rtype == "A" and not isinstance(v, ipaddress.IPv4Address): raise ValueError("That's an IPv6 address.")
 			if rtype == "AAAA" and not isinstance(v, ipaddress.IPv6Address): raise ValueError("That's an IPv4 address.")
-		elif rtype in ("CNAME", "TXT"):
+		elif rtype in ("CNAME", "TXT", "SRV"):
 			# anything goes
 			pass
 		else: