sock_util: Add unittest and fix detected issues.

Add additional checks to the port number parsing in str2ep to validate
the port number supplied in the string. This only verifies that the port
number is no longer than 5 chars and the resulting number fits in a
uint16_t.

It is still possible to supply up to 5 random chars.

Add a length check to the path to ensure that it fits in the supplied
buffer in the urlsplit function

Add missing null terminator to the returned host-port string in
sock_udp_ep_fmt

This adds a length check to verify if the host-port part of the URL fits
in the supplied buffer

URLs without a path were treated as invalid, while according to the URL
specification they are valid

Also fixes a missing null terminator in the returned path

Limit the number of scanned chars in _find_pathstart to the predefined
size

cflags.inc.mk: fix cflags test for llvm

lis3mdl: fix default params

In the default config .odr and .scale where switched around.

gcc does an error but not clang, so add -Werror.

atmega timer: Interrupt Pin

gnrc: mac: fix types according to print formatting

cflags: add -Wformat=2 -Wformat-overflow -Wformat-truncation

As reported by `llvm`.

lua: remove unsupported flags for llvm

tests: ignore infinite recursion for mpu_stack_test with clang

Prevents error:

  clang: error: unknown argument: '-fstack-usage'
  clang: error: unknown argument: '-fconserve-stack'

The flags are here to print debug information and try to optimize stack but are
not required.

w5100: fixed RX interrupt handling 

 A debug pin can be used to probe timer interrupts with an oscilloscope or
 other time measurement equipment. Thus, determine when an interrupt occurs
 and how long the timer ISR takes.
 The pin should be defined in the makefile as follows:

CFLAGS += -DDEBUG_TIMER_PORT=PORTF -DDEBUG_TIMER_DDR=DDRF \
            -DDEBUG_TIMER_PIN=PORTF4

stm32_common: fix ram length for MCUs with non-contiguous ram regions

makefiles/cflags.inc.mk: Use a template for CFLAGS testing

enc28j60: improvements to fix #9043

kw2xrf: Fix ack flag handling