Add fixme and default to none when srun and aprun are missing.

This can happen on an eslogin node. `aprun` isn't needed on the eslogin nodes because qsub "teleports" to the login node via ssh.

Without this fix, chpl_launcher.py will return a literal `None` when neither aprun or srun can be found. This causes `printchplenv` to throw an exception when run during the build process:

```bash
$ make
Traceback (most recent call last):
  File "/scratch1/scratchdirs/train36/chapel/util/printchplenv", line 194, in <module>
    _main()
  File "/scratch1/scratchdirs/train36/chapel/util/printchplenv", line 190, in _main
    print_mode(options.mode)
  File "/scratch1/scratchdirs/train36/chapel/util/printchplenv", line 125, in print_mode
    print_mode('launcher')
  File "/scratch1/scratchdirs/train36/chapel/util/printchplenv", line 72, in print_mode
    print_var('CHPL_LAUNCHER', launcher, mode, 'launch', ('launcher',))
  File "/scratch1/scratchdirs/train36/chapel/util/printchplenv", line 145, in print_var
    if not short_name or value.startswith(short_name):
AttributeError: 'NoneType' object has no attribute 'startswith'
Traceback (most recent call last):
  File "/scratch1/scratchdirs/train36/chapel/util/printchplenv", line 194, in <module>
    _main()
  File "/scratch1/scratchdirs/train36/chapel/util/printchplenv", line 190, in _main
    print_mode(options.mode)
  File "/scratch1/scratchdirs/train36/chapel/util/printchplenv", line 125, in print_mode
    print_mode('launcher')
  File "/scratch1/scratchdirs/train36/chapel/util/printchplenv", line 72, in print_mode
    print_var('CHPL_LAUNCHER', launcher, mode, 'launch', ('launcher',))
  File "/scratch1/scratchdirs/train36/chapel/util/printchplenv", line 145, in print_var
    if not short_name or value.startswith(short_name):
AttributeError: 'NoneType' object has no attribute 'startswith'
cd compiler && make
...
```