Add more checks for buffer overruns.

Add more checks for buffer overruns.
(XMALLOC_OVERRUN_CHECK_SIZE, xmalloc_overrun_check_header)
xmalloc_overrun_check_trailer, overrun_check_malloc)
overrun_check_realloc, overrun_check_free): Add.
(GC_STRING_EXTRA, string_overrun_pattern): Add.
(check_sblock, allocate_string_data, compact_small_strings):
Set and check string_overrun_pattern if GC_CHECK_STRING_OVERRUN.
(check_cons_list): Condition on GC_CHECK_CONS_LIST.
(check_string_free_list): Add.
(allocate_string, sweep_strings): Call check_string_free_list.