Implement user defined prefix.

Implement user defined prefix.

This patch adds the new 'define-prefix' command that creates (or mark an
existing user defined command) as a prefix command.
This approach was preferred compared to add a -prefix option to
'define' command : with define-prefix, a command can be defined and
afterwards marked as a prefix.  Also, it is easier to define a
'prefix' only command in one operation.

This patch also adds completers for the 'define' and 'document' commands.
This makes it easier for the user to type the prefixes for 'define'
and type the documented command name for 'document'.

gdb/ChangeLog
2019-11-30  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* cli/cli-script.c (do_define_command): Ensure a redefined
	prefix command is kept as a prefix command.
	(define_prefix_command): New function.
	(show_user_1): Report user defined prefixes.
	(_initialize_cli_script):  Create the new 'define-prefix' command.
	Add completers for 'define' and 'document'.
	* top.c (execute_command):  If command is a user-defined prefix only
	command, report the list of commands for this prefix command.