Improved assert messages

Improved assert messages

	* src/emacs.c: make the assertions_dont_abort a variable instead
	of define.

	* src/emacs.c (assert_failed): Make the 3rd nested assertion abort
	instead of exit. Add an extra newline in the 2nd nested
	assertion. Add a short backtrace of up to 32767 entries to the 2nd
	nested assertion. Add a backtrace to to the 1st assert_failed.

Signed-off-by: Nelson Ferreira <nelson.ferreira@ieee.org>