Finding the dump

[Stefan Monnier <monnier@IRO.UMontreal.CA>]

The new pdumper code is very welcome, but of course it introduces some
regressions and new problems:

- When I start my Emacs, it now says

    Loading loadup.el (source)...
    dump mode: nil
    [...]

  and goes on to (re)load the loadup.el instead of using the .pdmp file

  This is because I run my Emacs via a symlink, and load_pdump is not
  careful to try and follow symlinks while looking for the .pdmp next to
  the executable.  I think we should try and handle the use case rather
  than requiring the user to make a second matching symlink to the
  pdmp file.

- I wonder what distributions like Debian will say about having a .pdmp
  file in /usr/bin (AFAICT they normally only have executable files in
  there).  While we can let them hack their solution if they want to
  keep the dumps elsewhere, maybe we should directly add support for
  having dumps elsewhere since that might be useful in general.

  E.g., I think we should also search for the pdmp files in exec-directory.


        Stefan


PS: While "portable dumper" is a perfectly natural name for the code,
    I think the things we save aren't "portable dumps" (the code is
    portable but not the dump) but "heap images", so maybe the file name
    we use should reflect that.