slurm/interactive: Use `readonly` for immutable variables
slurm/interactive: Use `readonly` for immutable global variables
slurm/interactive: Clean up envsetup function

Move some variables to the top of the function.
slurm/interactive: Rename help function to "usage"
slurm/interactive: Use `readonly` for some more defaults
slurm/interactive: Use a here document for usage text
slurm/interactive: Remove old hard newline from usage text
slurm/interactive: Clean up usage function text

- Here document end anchor needs to be left aligned
- Text was indented too many levels
- Missing option for -J in example usage
- Clean up remaining echo I missed previously
slurm/interactive: Fix reference to old 'help' function
slurm/interactive: Add default job name (-J) to usage text
slurm/interactive: Remove envsetup call from options parser
slurm/interactive: Clean up logic for determining default SALLOC_OPTS

Use bash parameter expansion to use defaults if the user doesn't
specify a value.
slurm/interactive: Allow user to specify a node

This allows advanced users to specify which node their interactive
job goes to.