Fix the problems reported by prefix check of command-def-selftests.c

Fix the problems reported by prefix check of command-def-selftests.c

The next commit updates command-def-selftests.c to detect missing
or wrong prefix commands in a list of subcommands.
This command structure selftest detects a series of problems
that are fixed by this commit.

Many commands have a null prefix command, e.g.
    (gdb) maintenance selftest command_str
    Running selftest command_structure_invariants.
    list 0x560417949cb8 reachable via prefix 'append binary '.  command 'memory' has null prefixcmd
    list 0x560417949cb8 reachable via prefix 'append binary '.  command 'value' has null prefixcmd
    ...

Most of these are fixed by the following changes:
  * do_add_cmd searches the prefix command having the list
    in which the command is added.
    This ensures that a command defined after its prefix command
    gets the correct prefix command.
  * Due to the GDB initialization order, a GDB file can define
    a subcommand before the prefix command is defined.
    So, have add_prefix_cmd calling a new recursive function
    'update_prefix_field_of_prefix_commands' to set the prefix
    command of all sub-commands that are now reachable from
    this newly defined prefix command.  Note that this recursive
    call replaces the function 'set_prefix_cmd' that was providing
    a partial solution to this problem.

Following that, 2 python commands (defined after all the other GDB
commands) got a wrong prefix command, e.g. "info frame-filter" has
as prefix command the "i" alias of "info".  This is fixed by having
lookup_cmd_for_prefixlist returning the aliased command rather than
the alias.

After that, one remaining problem:
    (gdb) maintenance selftest command_str
    Running selftest command_structure_invariants.
    list 0x55f320272298 reachable via prefix 'set remote '.  command 'system-call-allowed' has null prefixcmd
    Self test failed: self-test failed at ../../classfix/gdb/unittests/command-def-selftests.c:196
    Ran 1 unit tests, 1 failed
    (gdb)

Caused by initialize_remote_fileio that was taking the address of
its arguments remote_set_cmdlist and remote_show_cmdlist instead
of receiving the correct values to use as list.

2020-05-15  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* cli/cli-decode.c (lookup_cmd_for_prefix): Return the aliased command
	as prefix, not one of its aliases.
	(set_cmd_prefix): Remove.
	(do_add_cmd): Centralize the setting of the prefix of a command, when
	command is defined after its full chain of prefix commands.
	(add_alias_cmd): Remove call to set_cmd_prefix, as do_add_cmd does it.
	(add_setshow_cmd_full): Likewise.
	(update_prefix_field_of_prefixed_commands): New function.
	(add_prefix_cmd): Replace non working call to set_cmd_prefix by
	update_prefix_field_of_prefixed_commands.
	* gdb/remote-fileio.c (initialize_remote_fileio): Use the real
	addresses of remote_set_cmdlist and remote_show_cmdlist given
	as argument, not the address of an argument.
	* gdb/remote-fileio.h (initialize_remote_fileio): Likewise.
	* gdb/remote.c (_initialize_remote): Likewise.