Add Gentoo specific changes

--- a/conf/example.conf.in
+++ b/conf/example.conf.in
@@ -170,6 +170,9 @@
 	# Example
 	# Accept every block device:
 	# filter = [ "a|.*|" ]
+	# Gentoo: we exclude /dev/nbd by default, because it makes a lot of kernel
+	# noise when you probed while not available.
+	filter = [ "r|/dev/nbd.*|", "a|.*|" ]
 	# Reject the cdrom drive:
 	# filter = [ "r|/dev/cdrom|" ]
 	# Work with just loopback devices, e.g. for testing:
@@ -1843,7 +1846,7 @@
 
 # Configuration section metadata.
 # This configuration section has an automatic default value.
-# metadata {
+metadata {
 
 	# Configuration option metadata/check_pv_device_sizes.
 	# Check device sizes are not smaller than corresponding PV sizes.
@@ -1888,7 +1891,8 @@
 	#
 	# This configuration option is advanced.
 	# This configuration option has an automatic default value.
-	# pvmetadatacopies = 1
+	# Gentoo: enable for data safety, but PV resize is then disabled.
+	# pvmetadatacopies = 2
 
 	# Configuration option metadata/vgmetadatacopies.
 	# Number of copies of metadata to maintain for each VG.
@@ -1931,7 +1935,7 @@
 	# This configuration option is advanced.
 	# This configuration option has an automatic default value.
 	# stripesize = 64
-# }
+}
 
 # Configuration section report.
 # LVM report command output formatting.