MEDIUM: peers: add the "localpeer" global option

MEDIUM: peers: add the "localpeer" global option

localpeer <name>
  Sets the local instance's peer name. It will be ignored if the "-L"
  command line argument is specified or if used after "peers" section
  definitions. In such cases, a warning message will be emitted during
  the configuration parsing.

  This option will also set the HAPROXY_LOCALPEER environment variable.
  See also "-L" in the management guide and "peers" section in the
  configuration manual.