NEW etc.: Replace most calls to safe_realloc_ary

NEW etc.: Replace most calls to safe_realloc_ary

Replace with REALLOC_ARY whenever the array object size is not 1

In subnet_list_set_len and ipset_set_len we abolish the unnecessary
temporary variable `nd'.  In subnet_list_set_len we also simplify the
assert integer overflow condition (the division is not needed because
REALLOC_ARY and hence safe_malloc_ary will check for potential
multiplication overflow).

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>