prefork: support --listen-address for consistency with forkserver

forkserver uses --listen-address to specify the listening socket address, while
prefork uses --interface.  Add the former as an alias for the latter, for
consistency & ease of integration.  The two still aren't commandline-compatible,
but they're fairly close.

Signed-off-by: Ask Bjørn Hansen <ask@develooper.com>

qpsmtpd-prefork

@@ -95,7 +95,8 @@ Usage: qpsmtpd-prefork [ options ]
 --quiet              : Be quiet (even errors are suppressed)
 --version            : Show version information
 --debug              : Enable debug output
---interface addr    : Interface daemon should listen on (default: $d_addr)
+--listen-address addr: Listen for connections on the address 'addr' (default: $d_addr);
+                       synonymous with --interface
 --port int           : TCP port daemon should listen on (default: $d_port)
 --max-from-ip int    : Limit number of connections from single IP (default: $maxconnip, 0 to disable)
 --children int       : Max number of children that can be spawned (default: $max_children)
@@ -115,7 +116,7 @@ GetOptions(
     'quiet'           => \$quiet,
     'version'         => sub { print "Qpsmtpd Daemon - version $VERSION\n"; exit 0; },
     'debug'           => \$debug,
-    'interface=s'     => \$d_addr,
+    'interface|listen-address=s' => \$d_addr,
     'port=i'          => \$d_port,
     'max-from-ip=i'   => \$maxconnip,
     'children=i'      => \$max_children,