treewide: use (x)reallocarray() when applicable

treewide: use (x)reallocarray() when applicable

reallocarray() prevents overflow of the multiplication.
It also avoids issues with operator precedence like in libmount/src/context.c:

    pids = realloc(cxt->children, sizeof(pid_t) * cxt->nchildren + 1);

This only allocated one additional byte, and not enough space for
another child.

Signed-off-by: Thomas Weißschuh <thomas@t-8ch.de>