66832e3a introduced change that prints core dumps in very detailed
format. That's completely out of user-friendliness but serves as a
measure for debugging hard-reproducible bugs.

The proper way to implement this:

  1. it must be controlled by command-line and environment variable;
  2. detailed traces must be default for buildbots only, for user
     invocations normal stack traces should be printed.

Options for control are: MTR_PRINT_CORE and --print-core that accept
the following values:

  no	         Don't print core
  short	       	 Print stack trace of failed thread
  medium	 Print stack traces of all threads
  detailed       Print all stack traces with debug context
  custom:<code>  Use debugger commands <code> to print stack trace

Default setting is: short (see env_or_default() call in pre_setup())

For environment variable wrong values are silently ignored (falls back
to default setting, see env_or_default()).

Command-line option --print-core (or -C) overrides environment
variable. Its default value is 'short' if not specified explicitly
(same env_or_default() call in pre_setup()). Explicit values are
checked for validity.

--print-method option can specify by which debugger we print
cores. For Windows there is only one choice: cdb. For Unix the values
are: gdb, dbx, lldb, auto. Default value is: auto

In 'auto' we try to use all possible debuggers until success.