pipe command completer

pipe command completer

This commit adds a completer for the "pipe" command.  It can complete
"pipe"'s options, and the specified GDB command.

To make the completer aware of the "-d" option, this converts the
option processing to use gdb::option.

Tests included.

gdb/ChangeLog:
2019-07-03  Pedro Alves  <palves@redhat.com>

	PR cli/24732
	* cli/cli-cmds.c (struct pipe_cmd_opts): New.
	(pipe_cmd_option_defs): New.
	(make_pipe_cmd_options_def_group): New.
	(pipe_command): Use gdb::option::process_options.
	(pipe_command_completer): New function.
	(_initialize_cli_cmds): Install completer for "pipe" command.

gdb/testsuite/ChangeLog:
2019-07-03  Pedro Alves  <palves@redhat.com>

	PR cli/24732
	* gdb.base/shell.exp: Load completion-support.exp.
	Adjust expected error output.  Add completion tests.