ls --color: do not colorize files with multiple hard links by default

ls --color: do not colorize files with multiple hard links by default

* src/ls.c: Rename hl->mh, do not colorize files with multiple
hard links by default.
* src/dircolors.c: Rename HARDLINK -> MULTIHARDLINK, hl -> mh.
* src/dircolors.hin: Do not colorize files with multiple hard links by
default.
* tests/Makefile.am: Rename the test case accordingly.
* tests/ls/multihardlink: Additionally test ls' default behavior
and factor out some duplication.
* NEWS: Mention the change in behavior.