[RFAv7 0/3] Allow the user to define default args for aliases

[Philippe Waroquiers <philippe.waroquiers@skynet.be>]

Allow the user to define default args for user-defined aliases.

This is version 7 of the patch series.
Compared to version 6, it implements the prevention of
defining an alias of an alias having default args, as suggested
by Simon.

This prevention is implemented in cli-cmds.c validate_aliased_command
(which is valid_command_p renamed and modified).

The default-args.exp test was enhanced to test this prevention.

In the documentation, the following sentence was added in @node Aliases:
@var{COMMAND} can also be the name of an existing alias.  In this case,
@var{COMMAND} cannot be an alias that has default arguments.


The version 6 of the patch series did the following changes,
compared to the previous version:
  * it handles the comment of Simon that default args should only
    be available for user-defined aliases and not for GDB commands
    and GDB pre-defined aliases.
    As default args are now only for user-defined aliases,
    the 'show/set enable-default-args [on|off]' commands have been
    removed.
  * documentation changed accordingly.
  * help and apropos now shows the full definition of the aliases having
    default args.

Previous versions handled the comments of Christian/Eli/Pedro/Tom/Simon.

This patch series implements, tests and documents the following commands:

  set default-args ALIAS [DEFAULT-ARGS...]
  show default-args [ALIAS]

It also changes the alias command to be:
  alias [-a] [--] ALIAS = COMMAND [DEFAULT-ARGS...]


Using the above default-args commands and arguments, you can
define default arguments for commands or define powerful aliases.

For example:

  alias bt_ALL = backtrace -entry-values both -frame-arg all -past-main -past-entry -full

defines the alias bt_ALL that will give as much information as possible
in a backtrace.

Default args can usefully be combined with the 'with' command, such as:

Make "wLapPeu" an alias of 2 nested "with":
  alias wLapPeu = with language pascal -- with print elements unlimited --

or have an alias pp10 to pretty print an expression with a maximum of
10 elements:

  alias pp10 = with print pretty -- with print elem 10 -- print

This patch series also adds a completer for 'alias'.

For what concerns changing 'alias -a' to 'alias -abbreviation":
This can for sure be done (and be backward compatible).  This can however
be done as a separate patch (I still also need to do a similar change
for the 'qcs' flags).