Reduce C lint

[ldoolitt]

Four commits; can be squashed or not.

Fix some C lint

• stringop-overflow in pico_icmp6.c
• stringop-truncation in pico_dev_tun.c pico_dev_tap.c
• maybe-uninitialized in pico_socket_ll.c pico_dev_radiotest.c
• sign-conversion in picoapp.c

(some of this only flagged by gcc when compiling with -O2)

Use standard and correct C initializers

• fixes missing-field-initializers

Get rid of stray ";" in pico_stack.h

• Makes most of code base pass c99 -pedantic
• pico_hotplug_detection.c is the exception; fails due to mixing function pointers with void pointers

Convert mock functions to use char*buf

• So we can legitimately do arithmetic on the pointers