strlcpy.h: Add STRLCPY() macro

strlcpy.h: Add STRLCPY() macro

It wraps strlcpy(3bsd) so that it performs some steps that one might
forget, or might be prone to accidents:

-  It calculates the size of the destination buffer, and makes sure it's
   an array (otherwise, using sizeof(dst) would be very bad).

-  It calculates if there's truncation, returning an easy-to-use value.

BTW, this macro doesn't have any issues of double evaluation, because
sizeof() doesn't evaluate its argument (unless it's a VLA, but then
the static_assert(3) within SIZEOF_ARRAY() makes sure VLAs are not
allowed).

Cc: Christian Göttsche <cgzones@googlemail.com>
Cc: Serge Hallyn <serge@hallyn.com>
Cc: Iker Pedrosa <ipedrosa@redhat.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>