fixed current_working_directory() to bypass static file links in path…

[JorySchossau]

… and return the real path

Fixes #6

get_current_dir_name() returns a path that follows symlinks which is different from all(?) other paths used in zupply. The difference created a problem where the relative paths returned for a file would contain multiple circuitous "../sameDirAgain" in a row. This fix replaces get_current_dir_name() with realpath() which circumvents symlinks.

This change did not break any more unittests than before (2 related to images are broken?)

[zhreshold]

Can you double check if realpath is always available in GNU?

[JorySchossau]

Does this matter? I only used it in this patch because it was being used elsewhere in zupply already.
To answer your question I'm not sure how to definitively check that, but I did find this:
"The advantage of using this function [realpath] is that it is more widely available [than the alternative canonicalize_file_name]. The drawback is that it reports failures for long path on systems which have no limits on the file name length."
from ftp://ftp.gnu.org/old-gnu/Manuals/glibc-2.2.3/html_chapter/libc_14.html

Also, get_current_dir_name has the extra stipulation that it is a GNU extension, whereas realpath does not, which I assume means it's part of basic GNU?