shell: refactor default shell command handling

before, we actually duplicated code for handling of shell commands
supplied to shell_init and those hardcoded by MODULE_SHELL_COMMANDS.

This patch refactors shell.c find_handler and print_help so there's
no duplicated code.
convert into host byte order in UDP reception
make TCP in destiny optional
udp_packet_handler uses printf, stack must be bigger
fix copy&paste typo
moved stack from header to C file
removed superfluous parameter in rpl_send()
 set message type for lowpan and ipv6 notifier
tiny debug improvement for RPL
added IPv6 upper-layer checksum calculation
calculate RPL checksum right before sending the ICMP packet
make use of the IPv6 upper-layer checksum calculation
fix debug output
fix some defines in tcb.h
catch some null pointers in RPL
provide a default next hop (preferred parent)
add toa to packets in nativenet

also change order of fields to reflect radio types header
fix recv_rpl_dio, missing return, refactor
add -Wextra -pedantic to native CFLAGS

it's all about code quality
remove all vtimers before (re)scheduliing it