Assume everywhere that the realpath() implementation has the
POSIX.1-2008 behaviour, i.e. allowing NULL as second parameter and thus
returning a newly allocated buffer; it is not just a GNU extension,
and supported already by modern libc's on other OSes.

menu_canonicalize_file_name() is always called with FALSE as second
parameter, so it is replaced directly by realpath(); this allows the
complete removal of canonicalize.{c,h}.