acme: Support DNS mode

acme: Support DNS mode

Tested with dynu.com ddns.

Install acme-dnsapi:

    # opkg install acme-dnsapi

Example `/etc/config/acme`:

    config acme
        option state_dir '/etc/acme'
        option account_email email@example.org'
        option debug '0'

    config cert 'foo'
        option enabled '1'
        option use_staging '1'
        option keylength '2048'
        option update_uhttpd '0'
        option dns 'dns_dynu'
        list domains 'foo.dynu.com'
        list domains '*.foo.dynu.com'
        list credentials 'Dynu_ClientId="xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"'
        list credentials 'Dynu_Secret="yyyyyyyyyyyyyyyyyyyyyyyyy"'

Run:

    # /etc/init.d/acme start

Signed-off-by: Zhong Jianxin <redacted>