By accident I was trying to open a non-existing database file and got
the following message:

	unable to open database file (unable to open database file)

which was not helpful.

By convention the error returned by a function should contain the
context passed to that function - the filename in Open case, the
operation performed and the error description. So

- teach OpenError to print filename of the opened database and the
  operation.
- don't duplicate the code in braces if we have the error message
  with the same text because they are often redundant.
- if we have only error code but not a message, still we have to include
  the filename and operation in the printed context.
- in the Open itself: always return OpenError on handling errors from
  sqlite3_open_v2 even if db == nil - instead of returning only Errno
  without any context.

Now the error in my case is:

	notfound.db: unable to open database file

( @gwenn requested to omit the ": open" (operation) probable because it
  duplicates error detail a bit )

/helped-by @gwenn

driver: Don't waste resources setting/being-notified up in progress handler if context cannot be canceled

driver: Don't waste resources setting/being-notified up in progress handler if context cannot be canceled

Please see https://github.com/mattn/go-sqlite3/pull/530 for similar
change. Here it is not additional goroutine but a callback setup which
will be called ever 100 ops. We can avoid that if we know that ctx
cannot be canceled.

Caveat: it will not be possible to retry on SQLITE_BUSY.

Add working cgo defaults for FreeBSD (#7)

To be fixed: cgocheck & panic

Still missing: StmtExecContext and StmtQueryContext

To be continued

It does not seem to be specify but it fails at runtime.