add %default-prompt-tag, and error (not abort()) on an abort to bad tag

add %default-prompt-tag, and error (not abort()) on an abort to bad tag

* libguile/init.c (scm_i_init_guile): Call scm_init_control after
  initing fluids.
* libguile/control.h (scm_sys_default_prompt_tag): New internal var.
* libguile/control.c (scm_c_abort): If abort is called for an unknown
  tag, raise an exception, except if the tag was the default prompt tag,
  in which case really abort -- to prevent recursion when some other
  patches land.
  (scm_init_control): Define %default-prompt-tag in the default
  environment.