ls: drop the dead code and the shadowed name

[metif12]

Follow-up to #201, from reading the notice list instead of filtering it.
Behaviour is unchanged.

• natural_compare held its shared part count in a local called max. That is
  the same shadowing that made six helpers in format_long.v uncompilable once
  max came from a module instead of being a builtin, and it is called as a
  function two lines away. It is shared now.
• The same function walked a three valued enum State to record one bit, so it
  is a bool now. Starting it false is what keeps a leading run of digits
  from emitting an empty first part, which is why the initial state was neither
  of the two real ones.
• Its comment restated the function name. It now says what -v changes.
• format_long.v had a local called checksum shadowing the checksum
  function, an unused statistics function that nothing called, and an unused
  options parameter on format_octal_permissions. Gone.
• icons.v and table.v carried two unused constants and an unused border
  function. They are part of the non-GNU extras, but dead is dead, and four
  notices in every build helps nobody.

v fmt -verify . passes, v run build.vsh builds all 76, v test . reports 49
of 49, and src/ls now builds with no notices at all.