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/orphans.c

@@ -120,7 +120,7 @@ cleanup(void)
 	}
 }
 
-prop_array_t
+prop_array_t SYMEXPORT
 xbps_find_orphan_packages(void)
 {
 	prop_array_t array;