some nice ADC changes... (warning: minor backward compat breakage)

- support for ADCs with unchecked arguments
  - rsync, htpasswd, and svnserve gone from core; turned into ADCs

Backward compat breakage and fix: Please see documentation for details,
but if you're using gitolite to control rsync you will now need to setup
ADCs (admin defined commands), and install at least the new "rsync" ADC.

----

Thanks to Joey Hess (see commit prior to this) for forcing me to stop
being lazy and get this out of my long term todo list.

contrib/adc/svnserve

@@ -0,0 +1,20 @@
+#!/usr/bin/perl
+
+use strict;
+use warnings;
+
+BEGIN {
+    die "ENV GL_RC not set\n" unless $ENV{GL_RC};
+    die "ENV GL_BINDIR not set\n" unless $ENV{GL_BINDIR};
+    unshift @INC, $ENV{GL_BINDIR};
+}
+use gitolite_rc;
+use gitolite;
+
+my $cmd = $ENV{SSH_ORIGINAL_COMMAND};
+
+die "expecting 'svnserve -t', got '$cmd'\n" unless $cmd eq 'svnserve -t';
+
+$SVNSERVE =~ s/%u/$ENV{GL_USER}/g;
+exec $SVNSERVE;
+die "svnserve exec failed\n";