rpl: fixed bug in debug msg introduced by my earlier changes
rpl: remove rpl_send_mutex

This PR is based on #2155.
Since RPL/Trickle is only using one thread in #2155, the send mutex becomes
obsolete and can be removed safely.
rpl: multiple instances

Conflicts:
	sys/net/routing/rpl/rpl_dodag.c
	sys/net/routing/rpl/rpl_storing/rpl_storing.c
rpl: multiple instances for nonstoring mode, too
multiple dodags for storing mode, mostly
check for root before deciding if to switch dodags
reset trickle timer on reception of DIS
send DIS before preferred parent is timed out
added check for instance id when deleting parents
check instance when deleting parent. more cleanup
rpl-p2p: add rdo support for storing mode
adapted rpl_udp example for rpl-p2p
temporary fix until PR #2190 is merged into master
rpl: wrong length of DIO options

Currently, the DIO options `dodag conf` and `prefix info` are off by two
bytes in their `length` field. The RFC states, that the length field
should not include the option `type` field and the `length` field (two bytes).

For Prefix Info Option: Option Length: 30 (RFC 6550, P.61)
For Dodag Conf  Option: Option Length: 14 (RFC 6550, P.52)

Wireshark complains about DIOs as malformed packets, otherwise.
Can be reproduced by running the rpl_udp example and logging the DIOs
via wireshark.
rpl: wrong length for DAO options

similar to #2216
According to RFC 6550, option `length` fields should not contain the two
bytes derived from the `length` field itself and the option `type` field.

Furthermore, the current implementation adds the parent's address (16
bytes) to transit options `length`, although this field is not used in
storing mode.
I subtracted the parennt's address length from the transit options
length for the storing mode.

Here again, can be reproduced by running the rpl_udp example and logging
DAOs via wireshark.