Previously, ngx_udp_rbtree_insert_value() was used for plain UDP and
ngx_quic_rbtree_insert_value() was used for QUIC.  Because of this it was
impossible to initialize connection tree in ngx_create_listening() since
this function is not aware what kind of listening it creates.

Now ngx_udp_rbtree_insert_value() is used for both QUIC and UDP.  To make
is possible, a generic key field is added to ngx_udp_connection_t.  It keeps
client address for UDP and connection ID for QUIC.

As per RFC 9000, Section 14:

  UDP datagrams MUST NOT be fragmented at the IP layer.

The rcf->studies list is unconditionally accessed by ngx_regex_cleanup(),
and this used to cause NULL pointer dereference if allocation
failed.  Fix is to set cleanup handler only when allocation succeeds.

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().