etc: Simplify default config by removing systemd sockets

Having systemd open sockets and pass them to chasquid is neat, but also
adds some complexity to the default config, for very little value in
practice.

This patch simplifies the default config by having chasquid open the
sockets instead.

Note that systemd file descriptor passing continues to be supported, and
existing installations will not be affected.

etc/systemd/system/chasquid.service

@@ -1,8 +1,5 @@
 [Unit]
 Description=chasquid mail daemon (service)
-Requires=chasquid-smtp.socket \
-	chasquid-submission.socket \
-	chasquid-submission_tls.socket
 
 [Service]
 ExecStart=/usr/local/bin/chasquid \
@@ -17,6 +14,9 @@ Restart=always
 User=mail
 Group=mail
 
+# Let chasquid listen on ports < 1024.
+AmbientCapabilities=CAP_NET_BIND_SERVICE
+
 # Simple security measures just in case.
 ProtectSystem=full