Candidate fix for [9f2eb3abac]: Have the whereShortCut() planner ignore indexes with more than four columns.
Restore fix [f15591f802], which was accidentally clobbered by the stat4 merge.
Never leave an open file descriptor pointing into the middle of the
database file if the file descriptor number is 2 or less.
Change the unix VFS so that it refuses to open a database file using
a file descriptor less than 3.
Make the unix VFS defensive against the error of having a database file open
on file descriptors 1 or 2, as an error message might easily be written onto
those file descriptors and thus overwrite and corrupt the database.
Improvements to the robust_open() logic in the unix VFS so that if an attempt
is made to open a repository on file descriptors 0, 1, or 2, and blocking
that file descriptor by opening it on /dev/null fails, then the open will
fail.