Fix up walkstk command

Summary: Two fixes to `walkstk`:

- If HPHP::mcg is corrupted, use a simple heuristic (is the
  corresponding native frame a normal, nameless frame?) to decide if
  we're in the TC.  This complicates the logic a tad but makes the
  command more reliable.

- Don't omit the initial frame.

Reviewed By: @markw65

Differential Revision: D1511373
Fix bugs in `hhx` for RATA immediates

Summary: I thought I fixed some of these in a previous diff, but apparently not.

Reviewed By: @alexmalyshev

Differential Revision: D1511765
Fix output buffer exception handling

Summary: Our ob_* system diverges from PHP5's in a number of ways.  This fixes
some or all of those issues, so that now we have the following:

- We prohibit calling ob_start() from inside an ob handler.

- If we throw from a handler, or from __toString() on its return value,
  rather than dropping the buffer's contents and swallowing the
  exception, we promote or print the buffer's contents and then throw.

Reviewed By: @markw65

Differential Revision: D1505602