create t/test_mod_userdir to replace sparse tests in tests/mod-userdir.t
remove tests/mod-userdir.t

buffer.[ch], chunk.[ch], request.[ch], sock_addr.[ch]

(might be "" if <backend>.server sets "fix-root-scriptname" => "enable")

NB: r->tmp_buf == srv->tmp_buf (pointer is copied for quicker access)

NB: request read and write chunkqueues currently point to connection
    chunkqueues; per-request and per-connection chunkqueues are
    not distinct from one another
      con->read_queue  == r->read_queue
      con->write_queue == r->write_queue

NB: in the future, a separate connection config may be needed for
    connection-level module hooks.  Similarly, might need to have
    per-request chunkqueues separate from per-connection chunkqueues.
    Should probably also have a request_reset() which is distinct from
    connection_reset().

when patching config

store pointer to module in handler_module instead of con->mode id

(self-referential)

NB: in the future, a separate connection state may be needed for
    connection-level state (different from request state)

NB: in the future, a separate plugin_ctx may be needed for
    connection-level plugins to keep state across multiple requests

move request start timestamps into (request_st *)

separate func to append log record to buffer

further isolate t/test_request.c tests to use (request_st *)

rename content_length to reqbody_length in request,
to more easily differentiate request body length
from response content_length

instead of (struct connection *)

con->proto_default_port is a property of the connection,
which influences the default port used in host normalization

(for memory locality and to reduce pointer chasing)

reorder some code
add more const
avoid redundant asserts

(when piped logger exited too quickly after being restarted
 and when server.max-worker = 0)

stat_cache.c no longer directly uses struct server *srv

fdevent.c no longer directly uses struct server *srv
srv->srvconf.max_fds (if set) is used to set rlimits
set max_conns in server.c after fdevent_init(), which sets srv->max_fds
  using srv->srvconf.max_fds (if set) as input hint

e.g. different server.errorlog for different virtual hosts

Also, support different server.breakagelog to have separate script
error logs, applicable to mod_cgi and mod_ssi exec.

employ chunk_buffer_acquire() and chunk_buffer_release()

separate log_error_open* funcs to open breakagelog, filename, syslog

pass http_parseopts arg (instead of con) to http_request_host_policy()

rather than using srv->tmp_buf directly in code modifying temp buf (tb)