csplit: improve integer overflow checking

csplit: improve integer overflow checking

* src/csplit.c: Prefer signed integers to unsigned for sizes
when either will do.  Check for some unlikely overflows.
(INCR_SIZE): Remove; no longer used.
(free_buffer): Also free the arg, simplifying callers.
(get_new_buffer): Use xpalloc instead of computing new
size by hand.  Add ATTRIBUTE_DEALLOC.
(delete_all_files, close_output_file):
If unlink fails with ENOENT, treat it as success.
(close_output_file): If unlink fails, decrement count anyway.
(parse_repeat_count, parse_patterns): Check for int overflow.
(check_format_conv_type): Use signed format.