vim should complain if viminfo is not writable

[Edward Z. Yang]

The situation is described quite well here: http://stackoverflow.com/a/10755253/23845

Steps to reproduce:
1. Create a new account with sudoer permission
2. Run 'sudo vim' (i.e. run vim as root, from the account)
3. Use vim and check if your command history is saved

Expected result: Command history is saved

Actual result: Command history is not saved. The ~/.viminfo file is owned by root and not readable/writable

Suggested fix: At the very least, vim should complain if it cannot read the .viminfo file. But ideally, 'sudo vim' would avoid creating a .viminfo file with the wrong permissions in the first place.

ProblemType: Bug
DistroRelease: Ubuntu 13.10
Package: vim 2:7.4.000-1ubuntu2
ProcVersionSignature: Ubuntu 3.11.0-14.21-generic 3.11.7
Uname: Linux 3.11.0-14-generic x86_64
NonfreeKernelModules: openafs
ApportVersion: 2.12.5-0ubuntu2.1
Architecture: amd64
Date: Wed Dec 11 13:09:07 2013
EcryptfsInUse: Yes
InstallationDate: Installed on 2013-11-21 (20 days ago)
InstallationMedia: Ubuntu 13.10 "Saucy Salamander" - Release amd64 (20131016.1)
MarkForUpload: True
SourcePackage: vim
UpgradeStatus: No upgrade log present (probably fresh install)