Pass thru clean via PROGS_TARGETS/SCRIPTS_TARGETS

Sponsored-by: EMC / Isilon Storage Division
Only deal with SCRIPTS once as part of bsd.prog.mk
Fix previous commit(s)

- Make FILES and SCRIPTS a dependency for all so they're built as
  part of the "many" case.
- Deal with PROGS being empty and SCRIPTS not being empty (this was the
  case that jmmv was originally trying to fix). This can be reproduced
  by going to tests/sys/pjdfstest, and just installing that directory
  (the previous commit wouldn't install kyua.sh and misc.sh).

Sponsored-by: EMC / Isilon Storage Division
Merge branch 'isilon-atf' into isilon-atf-fix-bsd-progs
Make sure errno is set to 0 before calling scalbn