New command line option -D.

	* NEWS: Mention it.
	* main.c (set_gdb_data_directory): New function.
	(captured_main): Recognize -D.  Flag error for --data-directory "".
	Call set_gdb_data_directory.
	(print_gdb_help): Print --data-directory, -D.
	* main.h (set_gdb_data_directory): Declare.
	* top.c (staged_gdb_datadir): New static global.
	(set_gdb_datadir): Call set_gdb_data_directory
	(show_gdb_datadir): New function.
	(init_main): Update init of data-directory parameter.

	testsuite/
	* gdb.base/catch-syscall.exp (test_catch_syscall_fail_nodatadir):
	Update.
	(do_syscall_tests_without_xml): Update.

	doc/
	* gdb.texinfo (Mode Options): Add -D.
* gdb.base/Makefile.in (EXECUTABLES): Add completion.
	* gdb.base/completion.exp: Check that all expected files exist
	before doing file completion.