How about this version?

It depends on a patch to libiberty.
I'll wait before that gets resolved before checking in of course.

I can think of one doc issue.
For command syntax in an @item existing usage would say to write something like

@item alias [-a] -f @var{from} -t @var{to}

instead of what I've written:

@item alias [-a] -f FROM -t TO

But I don't know how to properly refer to -f from in subsequent text
except as @samp{-f FROM} which is why I wrote what I wrote.
Suggestions welcome.

2011-09-21  Doug Evans  <dje@google.com>

        Add new "alias" command.
        * NEWS: Mention new command.
        * command.h (valid_user_defined_cmd_name_p): Declare.
        * cli/cli-decode.c (valid_user_defined_cmd_name_p): New function.
        * cli/cli-cmds.c (alias_command): New function.
        (init_cli_cmds): Add new command.

        doc/
        * gdb.texinfo (Extending GDB): Document alias command.

        testsuite/
        * gdb.base/alias.exp: Add tests for alias command.