RPL insert_srh_header: return success when no SRH is needed because the destination is a direct child

When it goes to the default label in the switch statement of
rpl_remove_header(), UIP_EXT_BUF does not always point to an IPv6 extension
header. "Move to next header" process should be done only in case of
UIP_PROTO_DESTO. Otherwise, it returns with doing nothing.

It falls into an infinite loop if it goes to the default label in the switch
statement of rpl_srh_get_next_hop() or rpl_process_srh_header().

According to RFC 2460 the length field of the Hop-by-Hop options header
should use a 8-octet unit (multiple of 8 byte).

In a normal configuration the RPL_HOP_BY_HOP_LEN define is 8, so the
current implementation works, but if RPL_HOP_BY_HOP_LEN is a multiple
of 8 the length is not calculated correctly.

rpl_verify_header: do not select DAG in storing mode after updating neighbor rank, as this may result in a No-Path DAO being sent, which will drop the current packet.

Enhanced RPL loop repair: send unicast DIO to source when detecting a loop from ext header, update rank along the path (when rank error is signaled)

Followed by doxyen file and group tags.
This patch is only a first attempt to make provide a clean structure,
many more files require rework, too.