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/humanize_number.c

@@ -39,7 +39,7 @@
 
 #include <xbps_api.h>
 
-int
+int SYMEXPORT
 xbps_humanize_number(char *buf, size_t len, int64_t bytes,
     const char *suffix, int scale, int flags)
 {