Add user_config example plugin

Add a plugin to read qpsmptd-style configuration files from users' home
directories.  Little to no testing yet.

plugins/user_config

@@ -0,0 +1,17 @@
+#!/perl (Editor hint)
+use strict;
+use warnings;
+
+sub hook_user_config {
+    my ($self,$txn,$user,$conf) = @_;
+    my $username = $user->host or return DECLINED;
+    my $filename = "/home/$username/.qpsmtpd/$conf";
+    return DECLINED unless -f $filename;
+    my $fh;
+    unless (open $fh,$filename) {
+        $self->log(LOGNOTICE,"Couldn't open $filename:$!");
+        return DECLINED;
+    }
+    map {chomp} (my @return = (<$fh>));
+    return OK,@return;
+}