userauth: don't register no-op auth functions and unify public API

[charlievieth]

This commit changes Open to not register any of the auth functions when userauth is disabled. It also unifies the public API of this library for when userauth is enabled/disabled.

NOTE: This PR builds on and includes https://github.com/mattn/go-sqlite3/pull/1302

goos: darwin
goarch: arm64
pkg: github.com/charlievieth/go-sqlite3
cpu: Apple M4 Pro
                       │   y1.txt    │               y3.txt                │
                       │   sec/op    │   sec/op     vs base                │
Suite/BenchmarkOpen-14   13.76µ ± 1%   12.37µ ± 1%  -10.11% (p=0.000 n=10)

                       │   y1.txt   │               y3.txt               │
                       │    B/op    │    B/op     vs base                │
Suite/BenchmarkOpen-14   712.0 ± 0%   111.0 ± 1%  -84.41% (p=0.000 n=10)

                       │   y1.txt    │               y3.txt               │
                       │  allocs/op  │ allocs/op   vs base                │
Suite/BenchmarkOpen-14   20.000 ± 0%   3.000 ± 0%  -85.00% (p=0.000 n=10)

[charlievieth]

@mattn / @rittneje could either of you take a look at https://github.com/mattn/go-sqlite3/pull/1302 and this PR? This PR depends on https://github.com/mattn/go-sqlite3/pull/1302.

[mattn]

Looks good to me. @rittneje What do you think?

[rittneje]

@mattn I think #1302 is supposed to go first, as these changes include those changes.

[charlievieth]

@mattn I think https://github.com/mattn/go-sqlite3/pull/1302 is supposed to go first, as these changes include those changes.

Correct, we should ideally merge https://github.com/mattn/go-sqlite3/pull/1302 before this PR (it's not strictly necessary since this PR contains the commit https://github.com/mattn/go-sqlite3/pull/1308/commits/83213c4e1a9696d43e32eac967f776b50d54ed41 from PR but for the sake of documentation and having commits tied to GH PRs it would probably best to merge that one first).

Thank you both for taking the time to review this!