tools/stackcount: Fix address resolution for user-space stack

This patch fixes issue #2748. The bug was that address-to-symbol resolution
didn't work for user-space stacks without the `-P` (per-pid) flag when tracing
single, isolated processes. The current documentation of the `-P` option
indicates that it's used to "display stacks separately for each process",
and this doesn't match with the intended usage.

This patch has two parts:

 - Fix `tools/stackcount.py` to explicitly set perpid to True if `-p <pid>`
   is used
 - Remove the `-P` option from the example of tracing single, isolated process
   in `tools/stackcount_example.txt`, since the usage of the option can
   be confusing (and unnecessary after the current change)