Add support for VxWorks (v3)

[Joel Brobecker <brobecker@adacore.com>]

Hello,

This is version 3 of the series to add support for VxWorks.
I think I addressed all the comments I received so far
(thank you!):

  - I moved the "command_post" observer, and then renamed it
    to "toplevel_command_post".  It seemed clearer that way.
    I'm not completely sure that users are going to like the fact
    that partition switch notices are no longer printed when that
    happens inside a script or a command, but I think that'll be OK,
    and I can deal with that then if they comment on it.

  - Thanks to Tom's suggestion about how to parse a list in TCL (thanks!)
    I could reduce a bit the number of parsing routines I'm adding to
    cli/cli-utils.

  - New module gdb_dlfcn (provides a platform-indenpendent dlopen/dlsym)

  - Changed the name of a few commands, as suggested.

  - Simplified the code in rs6000-vxworks-tdep.c, as per Mark's comments.

  - Documentation fixes, already reviewed by Eli.

I haven't answered Tom's question regarding "partitions" vs "program
and address spaces". I haven't completely designed what should be the
proper implementation for partitions. But I think it will most likely
use those concept. But it'll need some extensions, which I need to
formalize, when I have time.

I'd like to commit this after a reasonable wait, if all comments have
been answered...