Enhance the progress handler so that it keeps track of the number of VDBE
cycles across sqlite3_step() calls and issues callbacks when the cumulative
instruction count reaches threshold.
Fix a typo in main.c: SQLITE_DEAULT_AUTOMATIC_INDEX -> SQLITE_DEFAULT_AUTOMATIC_INDEX
In the command-line shell, work around the fact that popen() and pclose()
are not defined in stdio.h. in C89 and later.