bugfix: Resolve issue with incorrect self URL when using symlinks.

Recent commits have introduced a new way to obtain the self URL, honouring whatever is specified in 'baseurlpath'. However, this new code breaks when accessing SimpleSAMLphp through a path containing symbolic links in the file system, since the base directory refers always to the real path while the $_SERVER contents reflect what the web server sees (symlinks included). We use realpath() to convert a path with symlinks to a canonical path that we can compare.