VMS updates from KevinS (1 of 2: core)

VMS updates from KevinS (1 of 2: core)

His changes add file_exists() to sys/vms/vmsfiles.c, which might be
needed in the future but is not useful here.  Checking for 'gbd' and
'grep' for PANICTRACE support (which I had working on Alpha--and
ought to work on VAX, no idea about IA64--before losing access to vms)
is pointless since neither is used.

src/files.c
  SYSCF support, parse_config_line():
    #if VMS, ignore GDBPATH and GREPPATH if present in sysconf;

  SYSCF support, assure_syscf_file():
    #if VMS, force open()'s optional third argument since the macro
    which redirects open to vms_open requires it.

src/sp_lev.c
  selection_do_randline():
    use configuration-specified 'Rand()' rather than raw 'rand()'.