Resolve paths from perspective of config file rather than working directory

[rubenchristoffer]

Currently relative paths are resolved from the perspective of the working directory and not the actual config file. That can lead to trouble when specifying a config file that resides in a different directory than the working directory.

Here's an example of the problem:
Working directory is "/home/user".
Config file is "/home/user/config/config.linker"

If the config has a relative target path "./linkhere" and you run the command mklinker linkall --path /home/user/config/config.linker, then the actual link will be created in "/home/user/linkhere" rather than "/home/user/config/linkhere".