Implemented support for working with remote repositories.

libfetch from NetBSD's pkgsrc has been imported into lib/fetch, but
the objects are embedded into libxbps. Only a public function to fetch
files has been implemented: xbps_fetch_file().

The library now is built with -fvisibility=hidden by default, and
exported symbols are the ones that use the SYMEXPORT macro.

The code works well enough, but will need many more cleanups.

--HG--
extra : convert_revision : xtraeme%40gmail.com-20091027004600-0lq9aao67lisbzxv

lib/sortdeps.c

@@ -54,7 +54,7 @@ find_sorteddep_by_name(const char *pkgname)
 	return sdep;
 }
 
-int
+int SYMEXPORT
 xbps_sort_pkg_deps(prop_dictionary_t chaindeps)
 {
 	prop_array_t sorted, unsorted, rundeps;