VMS: Exit and exported symbol fixes.

[John E. Malmberg]

Resubmission against master.

This patch fixes the exit/child status handling code on VMS to properly 
use the UNIX source path, and removes the older VMS code that was not 
working properly.

It also adds VMS run-time options for selecting using the MCR command 
instead of a VMS foreign command or the argv[0] passed by a exec*() call.

The default is now set to use a foreign command when launched from DCL 
and the actual argv[0] when launched from exec().

It this mode, the macros created for the MAKE command look similar to 
what would normally be found in a makefile, ie: "make" or "/usr/bin/make"

In the MCR command mode, the macros contents will contain a space, such 
as "mcr dev:[dir]make".

The new default mode will also translate the make command properly 
regardless of the VMS CRTL feature settings, and is based on common code 
used in other ports of GNU projects to VMS.

This also makes the selection of using always using a command file 
(script) to run the child commands instead of individual commands an 
option and makes the default to not always use the script.  That feature 
was added for make 4.1.

This default was chosen because:

About 94 tests fail when make is set to always use a script.

About 44 tests fail in this new default mode, which was the behavior in 
make 4.0 and earlier.

Regards,
-John

0001-Bug-42447-VMS-simulate-exporting-symbols-Oct-6.patch
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