Switch to modern realpath()

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}.

This is a forward-port of the same changes done in gnome-menus, see:
https://gitlab.gnome.org/GNOME/gnome-menus/merge_requests/4

libmenu/Makefile.am

@@ -13,7 +13,6 @@ libmate_menu_include_HEADERS = \
 	matemenu-tree.h
 
 libmate_menu_sources = \
-	canonicalize.c \
 	desktop-entries.c \
 	entry-directories.c \
 	matemenu-tree.c \
@@ -23,7 +22,6 @@ libmate_menu_sources = \
 
 libmate_menu_la_SOURCES = \
 	$(libmate_menu_sources) \
-	canonicalize.h \
 	desktop-entries.h \
 	entry-directories.h \
 	matemenu-tree.h \