Multi headers are now using linked lists instead of arrays.  Notably,
the following fields were changed: r->headers_in.cookies (renamed
to r->headers_in.cookie), r->headers_in.x_forwarded_for,
r->headers_out.cache_control, r->headers_out.link, u->headers_in.cache_control
u->headers_in.cookies (renamed to u->headers_in.set_cookie).

The r->headers_in.cookies and u->headers_in.cookies fields were renamed
to r->headers_in.cookie and u->headers_in.set_cookie to match header names.

The ngx_http_parse_multi_header_lines() and ngx_http_parse_set_cookie_lines()
functions were changed accordingly.

With this change, multi headers are now essentially equivalent to normal
headers, and following changes will further make them equivalent.

Now a new structure ngx_proxy_protocol_t holds these fields.  This allows
to add more PROXY protocol fields in the future without modifying the
connection structure.

No functional changes.

Duplicate processing was possible if the address set by realip was
listed in set_realip_from, and there was an internal redirect so module
context was cleared.  This resulted in exactly the same address being set,
so this wasn't a problem before the $realip_remote_addr variable was
introduced, though now results in incorrect $realip_remote_addr being
picked.

Fix is to use ngx_http_realip_get_module_ctx() to look up module context
even if it was cleared.  Additionally, the order of checks was switched to
check the configuration first as it looks more effective.

Previously, the realip module could be left with uninitialized context after an
error in the ngx_http_realip_set_addr() function.  That context could be later
accessed by $realip_remote_addr and $realip_remote_port variable handlers.

Previously, when the client address was changed to the one from
the PROXY protocol header, the client port ($remote_port) was
reset to zero.  Now the client port is also changed to the one
from the PROXY protocol header.