PR3672 cont'd: Change pretty-print "depth" to mean substructures

PR3672 cont'd: Change pretty-print "depth" to mean substructures

We don't follow pointers anymore within a structure.  Now a single '$'
suffix means we print a shallow view of the structure, with {...} and
[...] in place of nested arrays/structs.  A double suffix '$$' means we
print everything contained in the struct.  Longer suffixes no longer
have meaning, and for now act the same as '$$'.

* tapsets.cxx (dwarf_pretty_print): Lose the "print_depth" field, and
  instead just check if this is a "full" print job.  Pass a bool
  indicating whether we're currently traversing the top-level, so we
  know which structs to expand.  Also wrap it all in try/catch so bad
  pointers don't kill us.