acme: Fix bugs from upstream's change to socat

The upstream acme.sh package changed to using socat instead of netcat;
update the dependencies to reflect this, and pass --listen-v6 when running
in standalone mode (since socat only listens on IPv4 by default).

Also add a missing cleanup call when certificate issuance fails.

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>

net/acme/Makefile

@@ -9,7 +9,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=acme
 PKG_VERSION:=2.7.9
-PKG_RELEASE:=1
+PKG_RELEASE:=2
 PKG_LICENSE:=GPLv3
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.gz
@@ -25,7 +25,7 @@ include $(INCLUDE_DIR)/package.mk
 define Package/acme
   SECTION:=net
   CATEGORY:=Network
-  DEPENDS:=+curl +ca-bundle +openssl-util +netcat
+  DEPENDS:=+curl +ca-bundle +openssl-util +socat
   TITLE:=ACME (Letsencrypt) client
 endef