A user could not override system-wide settings in their ~/.my.cnf,
because the DEFAULT_SYSCONFDIR was being searched last.  Also, in
some configurations (especially when the --sysconfdir compile-time
option is set to /etc or /etc/mysql), the system-wide my.cnf file
was read multiple times, causing confusion and potential problems.

Rearrange default directories to conform to the manual and logic.
Move --sysconfdir=<path> (DEFAULT_SYSCONFDIR) from the last default
directory to the middle of the list.  $HOME/.my.cnf should be last,
so the user is able to override the system-wide settings.

Change init_default_directories() to remove duplicates from the
list.


include/my_sys.h:
  Add array_append_string_unique(), from mf_arr_appstr.c
libmysql/Makefile.shared:
  Add new mf_arr_appstr.lo object
mysys/CMakeLists.txt:
  Add new mf_arr_appstr.c source.
mysys/Makefile.am:
  Add new mf_arr_appstr.c source.
mysys/default.c:
  Change order in which defaults files are added to default_directories,
  in order to conform to the manual (and to common sense).  This fixes
  a particularly bad problem on Unix, where ~/.my.cnf was read before
  /usr/local/etc/my.cnf.
  
  Also, don't add duplicate entries; move the existing entry to the
  end of the list instead.
  
  
  Here is a comparison of the order of defaults files, BEFORE and AFTER
  this patch.
  
  On Windows:
  
  BEFORE:  C:\, GetWindowsDirectory(), GetSystemWindowsDirectory(),
           $MYSQL_HOME, defaults-extra-file, INSTALLDIR
  
  AFTER:  GetSystemWindowsDirectory(), GetWindowsDirectory(), C:\,
          INSTALLDIR, $MYSQL_HOME, defaults-extra-file
  
  GetSystemWindowsDirectory() is moved before GetWindowsDirectory() because
  the former is shared by all Terminal Services users, while the latter is
  private for each user.
  
  
  On Netware (no change):
  
  BEFORE:  sys:/etc/, $MYSQL_HOME, defaults-extra-file
  
  AFTER:  sys:/etc, $MYSQL_HOME, defaults-extra-file
  
  
  On OS/2:
  
  BEFORE:  $ETC, /etc, $MYSQL_HOME, defaults-extra-file
  
  AFTER:  /etc, $ETC, $MYSQL_HOME, defaults-extra-file
  
  
  On everything else (general Unix):
  
  BEFORE:  /etc, $MYSQL_HOME, defaults-extra-file, ~/, --sysconfdir
  
  AFTER:  /etc/, --sysconfdir, $MYSQL_HOME, defaults-extra-file, ~/
  
  The BEFORE code added --sysconfdir on all systems, but only the
  Unix build system actually defined a value for it.
mysys/mf_arr_appstr.c:
  BitKeeper file /home/tsmith/m/bk/build/50-b20748/mysys/mf_arr_appstr.c