alloc-util: cleanups

alloc-util: cleanups

This patch contains a set of little cleanups for alloc-util.h:

1. The malloc_multiply(), realloc_multiply() and memdup_multiply()
functions check allocation related parameters on overflow. Let's
move them to the separate size_multiply_overflow() function for
simplicity, code duplication prevention and possible reuse in future.

2. use SIZE_MAX from stdlib instead of ((size_t) - 1) to be more
clear.

3. The 'a'/'b' variables are renamed to 'size' and 'need' to be
more clear.'