Add GUC backtrace_on_internal_error

Add GUC backtrace_on_internal_error

When enabled (default off), this logs a backtrace anytime elog() or an
equivalent ereport() for internal errors is called.

This is not well covered by the existing backtrace_functions, because
there are many equally-worded low-level errors in many functions.  And
if you find out where the error is, then you need to manually rewrite
the elog() to ereport() to attach the errbacktrace(), which is
annoying.  Having a backtrace automatically on every elog() call could
be very helpful during development for various kinds of common errors
from palloc, syscache, node support, etc.

Discussion: https://www.postgresql.org/message-id/flat/ba76c6bc-f03f-4285-bf16-47759cfcab9e@eisentraut.org