add proxy DNS server (by environment var)

[Robert Lu]

I find "proxyresolv" file is using DNS server 4.2.2.2, and I cannot modify it by proxychians.conf or environment vars.

So I submit this patch:

------start-------
diff -Nur A/proxychains-3.1/proxychains/proxyresolv B/proxychains-3.1/proxychains/proxyresolv
--- A/proxychains-3.1/proxychains/proxyresolv 2006-03-16 00:16:59.000000000 +0800
+++ B/proxychains-3.1/proxychains/proxyresolv 2015-04-17 17:57:01.591654631 +0800
@@ -2,7 +2,7 @@
# This script is called by proxychains to resolve DNS names

# DNS server used to resolve names
-DNS_SERVER=4.2.2.2
+DNS_SERVER=${PROXYRESOLV_DNS:-4.2.2.2}

if [ $# = 0 ] ; then
------end-------