Rework build_path() in slurmstepd to fix long-paths and spaces in names.

- Previously long paths/filenames were truncated at 256 characters.
  Use PATH_MAX instead.
- Function previously truncated the executable name at the first
  space character during own string copy function. Replace with
  strlcpy.
- Refactor to remove extraneous temp variables, use strlcpy
  instead of snprintf, getenvp instead of local env parser.