Tools directory location doesn't work when run via PATH

Looks like drrun locates the tools subdirectory by looking at argv[0], and if this is just a bare name (as it would be when executed via PATH) then it looks for the tools subdirectory relative to the current directory:

```
$ cwd /my/project
$ drrun -t drcov -- ls
ERROR: cannot find tool config file /my/project/../tools/drcov/drrun64
...
$ `which drrun` -t drcov -- ls
... works as expected ...
```

Using the --root option seems to have no effect.