dns cache per interface iteration 2

name server addresses are read from the dns
cache associated wih the interface on which
the request shall be done.

processes which has requested to issue dns request
using specific interface are now proxied to netd.

added methods to attach/detach a process to a specific
dns cache/interface.

added getaddrinfoforinface method which takes an
interface as an argument.

bug:4815099
bug:5465296
Change-Id: I7a8fe1980cdf99d4d296ddc5c6411f0c72162263

libc/docs/OVERVIEW.TXT

@@ -282,16 +282,9 @@ DNS resolver:
 
      - read /system/etc/resolv.conf instead of /etc/resolv.conf
 
-     - read the list of servers from system properties. the code looks for
-       'net.dns1', 'net.dns2', etc.. Each property should contain the IP
-       address of a DNS server.
-
-       these properties are set/modified by other parts of the Android system
-       (e.g. the dhcpd daemon).
-
-       the implementation also supports per-process DNS server list, using the
-       properties 'net.dns1.<pid>', 'net.dns2.<pid>', etc... Where <pid> stands
-       for the numerical ID of the current process.
+     - get the list of servers and the search domains for this process's
+       current interface from the dns cache.  This information is sent
+       from the framework via cache functions.
 
      - when performing a query, use a properly randomized Query ID (instead of
        a incremented one), for increased security.