Add "ls --hide".

Add "ls --hide".
(file_ignored): Renamed from file_interesting, with
inverted return value.  Accept the file name, not a struct dirent *.
All uses changed.  Avoid the expense of calling fnmatch if the
file is ignorable due to leading '.'.
(all_files, really_all_files): Removed; replaced by:
(ignore): New variable.  All uses changed.
(IGNORE_DEFAULT, IGNORE_DOT_AND_DOTDOT, IGNORE_MINIMAL, HIDE_OPTION):
New constants.
(hide_patterns): New variable.
(long_options, decode_switches, file_ignored, usage):
Add support for --hide.
(patterns_match): New function.
(usage): Replace "hide" with "ignore" in explanation, to avoid
confusion.