MXS-581: Moved to MD5 based encryption of passwords

The admin users are now encrypted with a more complex algorithm which reduces
the chance of two passwords generating the same hash by a significant amount.

Refer to the GNU libc manual for more details about the crypt function:
http://www.gnu.org/software/libc/manual/html_node/crypt.html

server/include/adminusers.h

@@ -31,7 +31,7 @@
  */
 #include <dcb.h>
 
-#define ADMIN_SALT "MS"
+#define ADMIN_SALT "$1$MXS"
 
 extern int  admin_verify(char *, char *);
 extern char *admin_add_user(char *, char *);