clamav: allow configuration to use TCP socket

Something having to do with passing a file descriptor over spamd's
Unix socket causes the ClamAV milter to fail. The milter says "ERROR:
Unknown reply from clamd," and running strace on spamd reveals "No file
descriptor received. ERROR."

Some work by others can be found on the Internet that suggests using
a TCP socket for the communication between the milter and spamd fixes
this. Lucian Cristian confirmed this on OpenWrt.

I am not sure why the Unix socket does not work. I suspect it is something
related to musl, but I have not yet found evidence of this.

This merge request adds the option to configure spamd to use a TCP
socket, and it uses this as the default. The merge request also adds an
init script for clamav-milter.

Signed-off-by: W. Michael Petullo <mike@flyn.org>

net/clamav/files/clamav-milter.config

@@ -0,0 +1,26 @@
+config clamav-milter 'clamav-milter'
+	option clamav_milter_config_file '/etc/clamav/clamav-milter.conf'
+	option Foreground 'true'
+	option PidFile '/var/run/clamav/clamav-milter.pid'
+	option User 'nobody'
+	option MilterSocketGroup 'nogroup'
+	option AllowSupplementaryGroups 'true'
+	option ReadTimeout '120'
+	option OnClean 'Accept'
+	option OnInfected 'Quarantine'
+	option OnFail 'Defer'
+	option AddHeader 'Replace'
+	option LogVerbose 'true'
+	option LogTime 'true'
+	option LogSyslog 'true'
+	option LogFacility 'LOG_LOCAL6'
+	option LogInfected 'Full'
+	option LogClean 'Basic'
+	option MaxFileSize '25M'
+	option SupportMultipleRecipients 'true'
+	option RejectMsg 'Rejecting Harmful Email: %v found.'
+	option TemporaryDirectory '/tmp'
+	option MilterSocket 'unix:/var/run/clamav/clamav-milter.sock'
+	option MilterSocketMode '666'
+	option ClamdSocket 'tcp:127.0.0.1:3310'
+	option FixStaleSocket 'true'