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/purge.c

@@ -33,7 +33,7 @@
 
 static int	remove_pkg_metadata(const char *);
 
-int
+int SYMEXPORT
 xbps_purge_all_pkgs(void)
 {
 
@@ -71,7 +71,7 @@ xbps_purge_all_pkgs(void)
  * This removes configuration files if they weren't modified,
  * removes metadata files and fully unregisters the package.
  */
-int
+int SYMEXPORT
 xbps_purge_pkg(const char *pkgname, bool check_state)
 {
 	prop_dictionary_t dict;