Add extra parameter to slurm_addto_char_list().

Rather than replace all existing references, keep the existing function
name and signature, and break off the body into
slurm_addto_char_list_with_case() with the lower_case_normalization
option set to true.

Locations that need to disable this case normalization will thus have
access to this same function, just by the longer name with the extra
boolean.