Allow blacklist to be disabled

There might be multiple clients using a single IP (e.g. NAT), which
can make the blacklist do more harm than good. So add a setting to
disable it if needed.
diff --git a/unix/x0vncserver/x0vncserver.man b/unix/x0vncserver/x0vncserver.man
index 2e6edc4..b54fcb4 100644
--- a/unix/x0vncserver/x0vncserver.man
+++ b/unix/x0vncserver/x0vncserver.man
@@ -142,6 +142,11 @@
 See the GnuTLS manual for possible values. Default is \fBNORMAL\fP.
 .
 .TP
+.B \-UseBlacklist
+Temporarily reject connections from a host if it repeatedly fails to
+authenticate. Default is on.
+.
+.TP
 .B \-BlacklistThreshold \fIcount\fP
 The number of unauthenticated connection attempts allowed from any individual
 host before that host is black-listed.  Default is 5.