DirWalker doesn't properly resolve symlinks that use relative paths

[stensonowen]

When smllr encounters a symbolic containing a relative path, it evaluates the absolute path as if the symlink were in env::current_dir(). Instead it should evaluate the path from the location of the link.

Steps to reproduce:

1. dd if=/dev/urandom of=foo bs=4K count=1
2. mkdir bar
3. cd bar
4. ln -s ../foo foo_symlink
5. cd ..
6. RUST_LOG=warn ./smllr .

WARN:smllr::walker: Couldn't get metadata for "../gettysburg_addr": No such file or directory (os error 2)