macro: add DISABLE_WARNING_SHADOW

As it turns out, we cannot use _Pragma in compound-statements. Therefore,
constructs like MIN(MAX(a, b), x) will warn due to shadowed variable
declarations. The DISABLE_WARNING_SHADOW macro can be used to suppress
these.

Note that using UNIQUE(_var) does not work either as GCC uses the last
line of a macro-expansion for __LINE__, therefore, still causing both
macros to have the same variables. We could use different variable-names
for MIN and MAX, but that just hides the problem and still fails for
MIN(something(MIN(a, b)), c).

The only working solution is to use __COUNTER__ and pass it pre-evaluated
as extra argument to a macro to use as name-prefix. This, however, makes
all these macros much more complicated so I'll go with manual
DISABLE_WARNING_SHADOW so far.
util: fix multiply-alloc helpers with size==0

Passing 0 to malloc() is not required to return NULL. Therefore, don't
bail out if "b" is 0. This is not of importance to the existing helpers,
but the upcoming realloc_multiply() requires this. To keep consistence, we
keep the same behavior for the other helpers.
util: add realloc_multiply() helper

This is similar to malloc_multiply() and friends. It is realloc() with a
multiplication-overflow check.
Reset signal-mask on re-exec to init=..

Process 1 (aka init) needs to be started with an empty signal mask.
That includes the process 1 that's started after the initrd is finished.
When the initrd is using systemd (as it does with dracut based initrds)
then it is systemd that calls the real init.  Normally this is systemd
again, except when the user uses for instance "init=/bin/bash" on the
kernel command line.