Fix regression from r289734 that caused crunchgen "subdirs" to not be
properly recursed.

The .for loop was defining a ${dir} variable that was being set at a
different evaluation time than the target itself, so every 'cd ${dir}'
became the last value that was in ${__dir}. This resulted in 'make obj'
not properly being ran in the tree that would leave .depend files
scattered around when 'make all' was ran in rescue/.

To fix this, define a CRUNCH_SRCDIR_* for every prog if it does not
already have one and then use that variable in every relevant place.
This allows simplifying some logic as well.

Reported by: emaste
X-MFC-With: r289734
MFC after: 3 weeks
Sponsored by: EMC / Isilon Storage Division