* backtrace.c (scm_display_error_message): Introduced fancy

* backtrace.c (scm_display_error_message): Introduced fancy
printing with max level 7 and length 10.  (Purpose: avoid printing
gigantic objects in error messages.)

* print.c, print.h (scm_i_port_with_print_state): New function.

* print.c (scm_iprin1, scm_printer_apply,
scm_port_with_print_state): Use scm_i_port_with_print_state.
(scm_simple_format): Modified not to destroy print states.
(print_state_mutex): New mutex.
(scm_make_print_state, scm_free_print_state, scm_prin1):
Lock/unlock print_state_mutex.