Make `--top-level` the default behavior of `nix-locate`

[Ma27]

It happens pretty often to me that I'm trying to find out which packages contain a certain header file (e.g. unistd.h). In those cases I usually don't care at all about which e.g. FHS env has this header as well.

I can't remember a single time where this was actually helpful, so I'm questioning whether this is a sensible default. I'll use this patch in my own setup for now, but I thought I could suggest it here as well.

cc @RaitoBezarius @figsoda @bennofs

[Ma27]

Rebased onto latest master now. Friendly ping @k900 @RaitoBezarius

[doronbehar]

Looks good to me ! Waiting for this to get in...

[Frontear]

Bump, this would be nice to get in! I've already used it through a personal patch and it works like a charm.

EDIT: sorry for the cross-links, GitHub doesn't like that I force pushed to update the commit description.