config.go:56:6: func ConfigUri should be ConfigURI
driver.go:223:26: error strings should not be capitalized or end with punctuation or a newline
sqlite.go:225:2: const OpenUri should be OpenURI
stmt.go:335:1: comment on exported var NullIfEmptyString should be of the form "NullIfEmptyString ..."

readme: Add link to another emerging sqlite library

https://crawshaw.io/blog/go-and-sqlite describes the rationale, but it
overlaps with github.com/gwenn/gosqlite in many places.

Switch to pkg-config to detect libsqlite3 CFLAGS/LDFLAGS

fixup! open: Fix error message

This commit fixes 0d47edb6. Recently I hit the following: I was trying
to open the database via

	sqlite3.Open(dburl, sqlite3.OpenNoMutex)

i.e. by passing flags, but erroneously no OpenReadWrite or something
similar, and got

	"testdata/1.sqlite: bad parameter or other API misuse"

as the error.

The message was confusing because there is no "open" action in it, so
add the action to OpenError and the mesage becomes:

	"testdata/1.sqlite: open: bad parameter or other API misuse"

See full discussion for structure of error messages here:

https://github.com/gwenn/gosqlite/pull/14

Recently I was playing with custom builds of libsqlite3 and neither
-lsqlite3 without other options find the library, not the compilation at
all works because libsqlite3 headers cannot be found.

There is established way to make this all working: by using pkg-config
builder can query it for a "package" cflags or libs e.g. this way:

	$ pkg-config --cflags sqlite3
	-I/home/kirr/local/sqlite/include

	$ pkg-config --libs sqlite3
	-L/home/kirr/local/sqlite/lib -lsqlite3

and use so-provided flags for the build.

And if the build is not custom pkg-config will just report cflags/libs
for a system package:

	$ pkg-config --cflags sqlite3

	$ pkg-config --libs sqlite3
	-lsqlite3

Since CGo provides support for pkg-config out of the box [1] we can use
it to get proper libsqlite3 CFLAGS/LDFLAGS.

I guess FreeBSD flags (added in e90ca0e4) becomes also not needed,
because by default pkg-config searches for package descriptions in
/usr/local/{lib,share}/pkgconfig too, so I'm switching that to
pkg-config too.

We leave explicit `-lsqlite3` to be there for non-(linux or freebsd) to
support Windows and Mac where pkg-config is probably not available by
default.

[1] https://golang.org/cmd/cgo/#Shdr-Using_cgo_with_the_go_command

open: Fix error message

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)