Previously, ngx_http_upstream_process_header() might be called after
we've finished reading response headers and switched to a different read
event handler, leading to errors with gRPC proxying.  Additionally,
the u->conf->read_timeout timer might be re-armed during reading response
headers (while this is expected to be a single timeout on reading
the whole response header).

Previously, ngx_http_upstream_test_next() used an outdated condition on
whether it will be possible to switch to a different server or not.  It
did not take into account restrictions on non-idempotent requests, requests
with non-buffered request body, and the next upstream timeout.

For such requests, switching to the next upstream server was rejected
later in ngx_http_upstream_next(), resulting in nginx own error page
being returned instead of the original upstream response.

Both Solaris and Windows define "s_addr" as a macro.

- use normal prefixes for types and macros
- removed some macros and types
- revised debug messages
- removed useless check of ngx_sock_ntop() returning 0
- removed special processing of AF_UNSPEC

The protocol used on inbound connection is auto-detected and corresponding
parser is used to extract passed addresses.  TLV parameters are ignored.

The maximum supported size of PROXY protocol header is 107 bytes
(similar to version 1).

Now it's clear from log error message if the error occurred on client or
upstream side.

All cases are harmless and should not happen on valid values, though can
result in bad values being shown incorrectly in logs.

Found by Coverity (CID 1430311, 1430312, 1430313).