Fix bs-nl handling, exit and Environment for VMS.

Fix bs-nl handling, exit and Environment for VMS.

This fix required a complete rewrite of the command parser vmsjobs.c
child_execute_job.  The old parser had too many incorrect assumptions
about DCL commands and could not be repaired to extended.

The parser now more closely parses VMS commands and handles quoted
commands and redirection.  Command File mode has been improved, but can
not fully support bs-nl syntax.

VMS Unix shell simulation has been improved.

* commands.c: vms_comma_separator is now a run-time setting.
* function.c: vms_comma_separator is now a run-time setting.
* function.c(func_basename_dir) now reports "[]" or "./" based on
  VMS crtl runtime setting.
* job.c(start_job_command): VMS Handle empty commands propery.
* main.c: Add VMS environment variables for run-time settings.
  * vms_legacy_behavior - Force older behavior.
  * vms_comma_separator - Commas or spaces for separators.
  * vms_unix_simulation - Enhanced Posix shell simulation features.
  * Detect if VMS CRTL is set to report Unix paths instead of VMS.
  * ':' and '>' are also MAP_DIRSEP on VMS.
* makeint.h: Add VMS run-time option variables.
* readme.vms: Update to current behavior.
* variable.c(define_variable_in_set): Fix VMS Environment variable
  lookup.
* variable.c(define_automatic_variables): Remove some VMS specific
  automatic variables and use the Unix ones instead.
* vms_export_symbol.c: Set max symbol size correctly.
* vmsjobs.c: child_execute_job() complete rewrite of VMS comand
  parsing.
* vmsjobs.c(build_vms_cmd): VMS commmand building with shell simulation.

Signed-off-by: Paul Smith <psmith@gnu.org>