Added functions pico_icmpX_ping_abort to ICMP4 and ICMP6

API to cancel ongoing ping operation (Fix #167)
Fixed unit test according to the new ICMPv4 API (return value of
pico_icmpv4_ping)
Added pico_dhcp_client_abort() function, to abort ongoing DHCP client
operation.

Fixed #168.
Refactoring of dhcp client internal timers to allow clean abort
Removed serialization of dhcp requests
Deep reworking of dhcp client

- removed internal "status"
- simplified initialization
- different static routes in ipv4 module for bcast addresses
Fixed comparison with unmatching signs
Fixed timer type in dhcpc
Re-enabled functional tests (disabled by mistake)
Enforced coding style + fixed ascii art in the mm state machine

Fixes #163
Fix possible free'd pointer dereference in dhcp_client. Possible fix for #171.

Waiting for RFtest results.
Reverted test modification (committed by mistake during fix for #171)
DHCP client improvement: use ROUTER option as default gateway
automatically
Typo in highport with udp only
Made a define for 1GB socket space (for testing purposes)

Possible fix for #176
Removed useless bitshifting in ipv6_nd (fixes #175)
Added "stm32f4xx" architecture in makefile
Added stm32f1xx architecture to makefile
Added "parameter problem" icmp message call for ICMPv6 - related to #173
Correct type (removed warning from previous commit)