slurmd: Always set conf->stepd_loc to slurmstepd path

Greatly simplify ability of code to get at current slurmstepd path
by setting slurmd's conf->stepd_loc to the default slurmstped path
if that path was not overridden on the command line.

This allows slurmd code to directly use conf->stepd_loc, instead of
requiring the duplicated code that created the default slurmstepd
path if conf->stepd_loc was not set at each call site.