Increase MAXNS to at least 4 as we are in age of dualstack systems

[Aminda Suomalainen]

Currently, according to resolv.comf manpage, it's 3 which might work with singlestack systems, but in dualstack systems ISPs generally have twoIPv4 nameservers and two IPv6 nameservers.

With the current MAXNS Ubuntu only receives three nameservers instead of four which the ISP has (meaning two IPv4 DNS servers and one IPv6 one while even Windows allows 2 two DNS servers per internet protocol version).

```
       nameserver Name server IP address
              Internet address of a name server that the resolver should
              query, either an IPv4 address (in dot notation), or an IPv6
              address in colon (and possibly dot) notation as per RFC 2373.
              Up to MAXNS (currently 3, see <resolv.h>) name servers may be
              listed, one per keyword. If there are multiple servers, the
              resolver library queries them in the order listed. If no name‐
              server entries are present, the default is to use the name
              server on the local machine. (The algorithm used is to try a
              name server, and if the query times out, try the next, until out
              of name servers, then repeat trying all the name servers until a
              maximum number of retries are made.)
```

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: libc6-dev 2.21-0ubuntu5
ProcVersionSignature: Ubuntu 4.3.0-5.16-generic 4.3.3
Uname: Linux 4.3.0-5-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.19.3-0ubuntu3
Architecture: amd64
CurrentDesktop: MATE
Date: Thu Jan 14 10:23:51 2016
InstallationDate: Installed on 2015-12-12 (32 days ago)
InstallationMedia: Ubuntu-MATE 16.04 LTS "Xenial Xerus" - Alpha amd64 (20151212)
SourcePackage: glibc
UpgradeStatus: No upgrade log present (probably fresh install)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.