MINOR: acme: add the acme section in the configuration parser

MINOR: acme: add the acme section in the configuration parser

Add a configuration parser for the new acme section, the section is
configured this way:

    acme letsencrypt
        uri https://acme-staging-v02.api.letsencrypt.org/directory
        account account.key
        contact foobar@example.com
        challenge HTTP-01

When unspecified, the challenge defaults to HTTP-01, and the account key
to "<section_name>.account.key".

Section are stored in a linked list containing acme_cfg structures, the
configuration parsing is mostly resolved in the postsection parser
cfg_postsection_acme() which is called after the parsing of an acme section.