libarchive: FEATURE_LZMA_FAST should be visible if FEATURE_SEAMLESS_LZMA

While at it, tweak some config help texts.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

networking/ping.c

@@ -59,7 +59,7 @@
 //config:	default y
 //config:	depends on FEATURE_IPV6
 //config:	help
-//config:	  This will give you a ping that can talk IPv6.
+//config:	  Alias to "ping -6".
 //config:
 //config:config FEATURE_FANCY_PING
 //config:	bool "Enable fancy ping output"