compare file names, not path names

the latter may be absolute, relative or symlinks
and realpath() is not portable enough
Revert "always merge arguments of "-file" parameter"

This reverts commit 4c3a863c63d9caa157afa86c26c070cfeba9ca4c.

Always using "-merge" may lead to undesired behaviour in WADs that are
not supposed to be loaded like this, e.g. av.wad. See for example the
issue in https://github.com/chocolate-doom/chocolate-doom/issues/436

reopens #20