Avoid /proc on BSDs as it may not be mounted

[jbeich]

Get path to executable via KERN_PROC_PATHNAME on BSDs, where /proc is not mounted by default. Let's implement getexecname() to avoid complicating

libbacktrace/fileline.c

Lines 86 to 114 in 5a99ff7

	for (pass = 0; pass < 5; ++pass)
	{
	int does_not_exist;
	switch (pass)
	{
	case 0:
	filename = state->filename;
	break;
	case 1:
	filename = getexecname ();
	break;
	case 2:
	filename = "/proc/self/exe";
	break;
	case 3:
	filename = "/proc/curproc/file";
	break;
	case 4:
	snprintf (buf, sizeof (buf), "/proc/%ld/object/a.out",
	(long) getpid ());
	filename = buf;
	break;
	default:
	abort ();
	}
	if (filename == NULL)
	continue;

Fixes rust-lang/rust#54434
These changes are in the public domain.

[ghost]

@ianlancetaylor Is there any chance to get this merged?

[ianlancetaylor]

Thanks. I fixed this in a different way, that seemed cleaner to me. Thanks for sending the patch.