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/cmpver.c

@@ -223,7 +223,7 @@ get_component(const char *position, version_component *component)
  * of the version should conform to the porting guidelines. It can contain
  * multiple components, separated by a period, including letters.
  */
-int
+int SYMEXPORT
 xbps_cmpver(const char *pkg1, const char *pkg2)
 {
     const char *v1, *v2, *ve1, *ve2;