MINOR: http-htx: Add a new section to create groups of custom HTTP errors

MINOR: http-htx: Add a new section to create groups of custom HTTP errors

A new section may now be declared in the configuration to create global groups
of HTTP errors. These groups are not linked to a proxy and are referenced by
name. The section must be declared using the keyword "http-errors" followed by
the group name. This name must be unique. A list of "errorfile" directives may
be declared in such section. For instance:

    http-errors website-1
        errorfile 400 /path/to/site1/400.http
        errorfile 404 /path/to/site1/404.http

    http-errors website-2
        errorfile 400 /path/to/site2/400.http
        errorfile 404 /path/to/site2/404.http

For now, it is just possible to create "http-errors" sections. There is no
documentation because these groups are not used yet.