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()
dao_counter wasn't initialized
informational output as debug
use a meaningful stacksize define
add missing stdlib.h include
removed unnecessary stdlib.h include
Merge pull request #354 from OlegHahm/network_cleanup

Network cleanup
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
fix table header indentation