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/fexec.c

@@ -128,7 +128,7 @@ vfcexec(const char *path, int skipempty, const char *arg, va_list ap)
 	return retval;
 }
 
-int
+int SYMEXPORT
 xbps_file_exec(const char *arg, ...)
 {
 	va_list	ap;
@@ -141,7 +141,7 @@ xbps_file_exec(const char *arg, ...)
 	return result;
 }
 
-int
+int SYMEXPORT
 xbps_file_exec_skipempty(const char *arg, ...)
 {
 	va_list	ap;
@@ -154,7 +154,7 @@ xbps_file_exec_skipempty(const char *arg, ...)
 	return result;
 }
 
-int
+int SYMEXPORT
 xbps_file_chdir_exec(const char *path, const char *arg, ...)
 {
 	va_list	ap;