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When a non-standard variable has a prefix of a standard variable,
apkbuild-lint does not complain at the moment. For example:

```sh
depends_foo="abc"
```

is a non-standard variable, but because it starts with `depends`, which
is a standard variable, it is not caught.

Let's fix that by including an `=` in the negative lookahead, so no
longer functions as a prefix match. See [regexp101 example][0].

[0]:https://regex101.com/r/qhVwsP/1

start_pre is perfectly fine to have in an init script, but initd-lint
accidentally flagged it as a custom `stop` function.

Tighten the match for custom start/stop function to prevent false
positives.

apkbuild-lint guards against parameters starting with a double
underscore as they are reserved. But the check for this is a bit too
eager and also warns when something starts with a double underscore
which is not a variable.

For example:

```sh
function foo() {
    make -D__VAR=value
}
```

is a legal construct, but this resulted in a warning. This is because
the start-of-string / tab part was marked optional, which makes the test
look for anything starting with a double underscore.

Remove the `?` from the group to actual anchor the check.

Fixes #24

Kevin Daudt authored Aug 26, 2019 and Leo committed Aug 26, 2019

The check for pkgname in the source url assumed the value started with
the url protocol. This is not the case when a custom archive name has
been specified.

Fix this by allowing an optional archive name followed by `::` before
the actual url.

Fixes #22