(struct link): Remove unused linkcount member.

(struct link): Remove unused linkcount member.
(base_64_digits): Move to list.c.
(base_8_digits): Remove.
(to_octal): New function, with some of old contents of to_base.
(to_base): Remove.
(to_base256): New function.
(to_chars): Use base 256, not base 64, for huge values.
(mode_to_chars): Don't use two's complement in GNU format or POSIX format.
(dump_file): Interchange last two arguments.  If TOP_LEVEL is negative,
it means we have an incremental dump where we don't know whether this
is a top-level call.
Use deref_stat instead of statx / stat / lstat.
Cast result of alloca.
Check for dates if 0 < top_level, not if listed_incremental_option.
Move multiple-link check after directory check.
Do not dump avoided names.
Dump hard links to symbolic names as links, not as separate symbolic links.
start_header cannot return a null pointer, so don't test for it.
Likewise for find_next_block.