This ensures that errors which happen during logging to syslog are logged
with proper context, such as "while logging to syslog" and the server name.

Prodded by Safar Safarly.

During initial startup the ngx_cycle->hostname is not available, and
previously this resulted in incorrect logging.  Instead, hostname from the
configuration being parsed is now preserved in the syslog peer structure
and then used during logging.

Similarly, ngx_cycle->log might not match the configuration where the
syslog peer is defined if the configuration is not yet fully applied,
and previously this resulted in unexpected logging of syslog errors
and debug information.  Instead, cf->cycle->new_log is now referenced
in the syslog peer structure and used for logging, similarly to how it
is done in other modules.

An UTF-8 octet sequence cannot start with a 11111xxx byte (above 0xf8),
see https://datatracker.ietf.org/doc/html/rfc3629#section-3.  Previously,
such bytes were accepted by ngx_utf8_decode() and misinterpreted as 11110xxx
bytes (as in a 4-byte sequence).  While unlikely, this can potentially cause
issues.

Fix is to explicitly reject such bytes in ngx_utf8_decode().

This is expected to help with clients using pipelining with some constant
depth, such as apt[1][2].

When downloading many resources, apt uses pipelining with some constant
depth, a number of requests in flight.  This essentially means that after
receiving a response it sends an additional request to the server, and
this can result in requests arriving to the server at any time.  Further,
additional requests are sent one-by-one, and can be easily seen as such
(neither as pipelined, nor followed by pipelined requests).

The only safe approach to close such connections (for example, when
keepalive_requests is reached) is with lingering.  To do so, now nginx
monitors if pipelining was used on the connection, and if it was, closes
the connection with lingering.

[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=973861#10
[2] https://mailman.nginx.org/pipermail/nginx-devel/2023-January/ZA2SP5SJU55LHEBCJMFDB2AZVELRLTHI.html

Casts are believed to be not needed, since memcmp() has "const void *"
arguments since introduction of the "void" type in C89.  And on pre-C89
platforms nginx is unlikely to compile without warnings anyway, as there
are no casts in memcpy() and memmove() calls.

These casts were added in 1648:89a47f19b9ec without any details on why they
were added, and Igor does not remember details either.  The most plausible
explanation is that they were copied from ngx_strcmp() and were not really
needed even at that time.

Prodded by Alejandro Colomar.