Minor RPL improvements

This pull request bundles a few minor RPL improvements and fixes a few issues in the comments and the debug output.

* The link metric was not converted to RPL ranks correctly, which could lead to a node other than the root to get a rank of 1.5 * RPL_MIN_HOPRANKINC. Its minimum allowed rank, however, would be 2 * RPL_MIN_HOPRANKINC. The simplest fix is to ensure that the ETX is calculated on the same scale as the rank.
* Fix problem with uninitialised memory in rpl-icmp6.c
* Make it possible to disable DIS transmission by setting RPL_DIS_SEND to 0.