MINOR: cfgparse-global: add cfg_parse_global_localpeer

MINOR: cfgparse-global: add cfg_parse_global_localpeer

This commit prepares the parsing of localpeer keyword in MODE_DISCOVERY. We
need this, as HAPROXY_LOCALPEER environment variable could be checked in the
configuration in order to enable some backend or frontend settings.

So, let's at first add a dedicated parser for localpeer. At second, we no
longer need to check, if cfg_peers is valid pointer, as in MODE_DISCOVERY we
parse only the "global" section.

In addition, let's make the code of localpeer parser a little bit more
readable.