clamav: allow configuration to use TCP socket

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>