task_repository: avoid syntax error if $FAI_ROOT/etc/resolv.conf is a dangling symlink

If $FAI_ROOT/etc/resolv.conf is pointing to a non-existing file like:

| % ls -la /etc/resolv.conf
| -rw-r--r-- 1 root root 140 Feb 14  2012 /etc/resolv.conf
| % ls -la [...]/etc/resolv.conf
| lrwxrwxrwx 1 root root 31 Sep 10 16:05 etc/resolv.conf -> /etc/resolvconf/run/resolv.conf
| % ls -la /etc/resolvconf/run/
| ls: cannot access /etc/resolvconf/run/: No such file or directory

and then we're running into:

| fcopy: Nonexisting directory /etc/grml/fai/config/files/etc/resolv.conf. No files copied.
| /usr/lib/fai/subroutines: line 948: [: /etc/resolv.conf: unary operator expected
| `/etc/resolv.conf' -> `/srv/jenkins/jobs/grml32-full_Release/workspace/grml_chroot/etc/resolv.conf'
| cp: not writing through dangling symlink `/srv/jenkins/jobs/grml32-full_Release/workspace/grml_chroot/etc/resolv.conf'

Avoid the syntax error by quoting the test accordingly.