Bug #20748: Configuration files should not be read more than once

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.

libmysql/Makefile.shared

@@ -59,7 +59,7 @@ mysysobjects1 =		my_init.lo my_static.lo my_malloc.lo my_realloc.lo \
 			mf_pack.lo my_messnc.lo mf_dirname.lo mf_fn_ext.lo\
 			mf_wcomp.lo typelib.lo safemalloc.lo my_alloc.lo \
 			mf_format.lo mf_path.lo mf_unixpath.lo my_fopen.lo \
-			my_symlink.lo my_fstream.lo \
+			my_symlink.lo my_fstream.lo mf_arr_appstr.lo \
 			mf_loadpath.lo my_pthread.lo my_thr_init.lo \
 			thr_mutex.lo mulalloc.lo string.lo \
                         default.lo default_modify.lo \