minor style change: fixed indentation

Added dummy sys_mbox_trypost to unit test port

Start implementing unit tests for timers

Make sys_arch_sem_wait() & sys_arch_mbox_fetch() a little more robust when waiting (although this should never be a problem...)

let unit test sys_arch check that a mutex is not taken recursively

sys_arch unit tests: refine test_sys_arch_waiting_fn a bit

hopefully fix building unit tests on linux/clang

tried to fix sockets unit tests; fix configuration to run with any NO_SYS setting and with/without IPv6 (IPv4 is required)

Fix and improve sockets unit test and unit test sys_arch (with a little help of tcpip.c)

tried to add basic socket unit tests (nonsense only for now); made LOCK_TCPIP_CORE()/UNLOCK_TCPIP_CORE() overridable for that