bug#34180: 27.0.50; argv[0] used incorrectly to find the .pdmp

[Eli Zaretskii]

> From: Daniel Colascione <dancol@dancol.org>
> Date: Sat, 26 Jan 2019 19:54:29 -0800
> 
> > 1- It fails miserably if argv[0] is a name relative to $PATH since it
> >     performs the lookup relative to $PWD instead, which is additionally
> >     a security issue.
> > 
> > 2- If the executable named by argv[0] is a symlink, it does not try to
> >     follow the symlink in case the .pdmp is stored next to the
> >     destination rather than next to the source.
> 
> Yep. We should definitely fix that. realpath on argv[0] seems like the 
> right thing.

Wouldn't it be better to have a method of finding the absolute file
name of the executable from which the process was started, regardless
of what argv[0] says?  The way to do that is system-dependent (on
GNU/Linux, I believe you need to readlink ("/proc/self/exe") or
somesuch, for example), but AFAIK this can be done on all platforms we
support.