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/remove.c

@@ -183,7 +183,7 @@ dirs:
 	return 0;
 }
 
-int
+int SYMEXPORT
 xbps_remove_pkg(const char *pkgname, const char *version, bool update)
 {
 	prop_dictionary_t dict;