Change os_unix.c to use either one or two mappings internally.
Modify various test cases so that they work if the file is extended in units of the system page-size.
Avoid assuming the page-size is 4096 bytes in os_unix.c.
Fix a problem in btree.c that could cause a crash following an OOM. Also various test script problems.
Further fixes for test scripts.
Minor changes to unixMapfile() function.