Summary:
As a security precaution, GHCi helpfully refuses to run a .ghci file if it is owned by another user. But if the that other user is root, then arguably GHCi should not refuse to interpret the file, because if root really was malicious, then the user would be having a bad day anyways.
This means that .ghci files installed in a global location, say under /usr/local/, can now be read.

Fixes #9324

Test Plan:
```
$ sudo touch .ghci
$ ghci
```
Notice that the warning about the file being owned by someone else is now gone.

Reviewers: austin

Reviewed By: austin

Subscribers: phaskell, simonmar, carter, nomeata, relrod

Projects: #ghc

Differential Revision: https://phabricator.haskell.org/D75