Fix another bug: ls --indicator-style=file-type would call

Fix another bug: ls --indicator-style=file-type would call
stat for a symlink, even though it wasn't always needed.
In some cases, that unnecessary stat would cause ls to fail.
* src/ls.c (gobble_file): Don't treat symlinks specially (in
requiring a stat syscall).  Remove the offending exclusion.
* NEWS: Mention the fix.
* tests/ls/stat-dtype: New file/test, for the above fix.
Also exercises the new df feature, below.