Modernize APKBUILD, update license, disable tests, add openrc subpkg

Several tests fail under abuild:

PASS: address_test
PASS: buffer_test
PASS: cfg_tokenizer_test
PASS: table_test
PASS: http_test
PASS: tls_test
PASS: binder_test
FAIL: connection_reset_test
PASS: bad_request_test
FAIL: fallback_test
FAIL: functional_test
FAIL: bind_source_test
FAIL: fd_limit_test
FAIL: ipv6_v6only_test
FAIL: proxy_header_test
SKIP: transparent_proxy_test
PASS: config_test
FAIL: reuseport_test
FAIL: reload_test
FAIL: slow_client_test
PASS: resolv_test
PASS: bad_dns_request_test

Running "make check" manually there's only one fail:

PASS: address_test
PASS: buffer_test
PASS: cfg_tokenizer_test
PASS: table_test
PASS: http_test
PASS: tls_test
PASS: binder_test
PASS: functional_test
PASS: bad_request_test
PASS: bind_source_test
PASS: connection_reset_test
PASS: fallback_test
PASS: fd_limit_test
PASS: ipv6_v6only_test
FAIL: proxy_header_test
PASS: reload_test
PASS: reuseport_test
PASS: slow_client_test
SKIP: transparent_proxy_test
PASS: config_test
PASS: resolv_test
PASS: bad_dns_request_test

Now all invocations have following order of arguments (if present):

    addgroup -S -g ... GROUP
    adduser -S -u ... -D -H -h ... -s ... -G ... -g ... USER

Following rules have been applied:
- script starts with shebang !#/bin/sh followed by blank line,
- script ends with exit 0 prepended by blank line,
- only stderr of adduser, addgroup or passwd is redirected to /dev/null,
- getent passwd/group instances has been removed,
- manual checking of file and group existence has been removed,
- `|| true` instances has been removed.

Comments and line wrapping have been preserved.

This way we can avoid ugly default:

    Linux user,,,

It's only for consistency and to ease spotting lack of it.