Do not silently update xbps on any install/update transaction.

When there's a new xbps update, xbps-install(1) will now return
EBUSY (16) and a message (if dry-run disabled) explaining
how to proceed.

If there's an update and transaction does not contain xbps, it will
error out unless the 'xbps' pkg is the only target pkg, i.e:

	# xbps-install -Su
	# echo $?
	16

To update xbps, the only way to proceed is to explicitly declare
it as an update, i.e:

	# xbps-install -u xbps

The dry-run mode will still show there's an xbps update.

Modified the existing test cases to satisfy the new behaviour.

Closes #166
Closes #142

include/xbps.h.in

@@ -50,7 +50,7 @@
  *
  * This header documents the full API for the XBPS Library.
  */
-#define XBPS_API_VERSION	"20190707"
+#define XBPS_API_VERSION	"20191227"
 
 #ifndef XBPS_VERSION
  #define XBPS_VERSION		"UNSET"