HP-UX needs _HPUX_ALT_XOPEN_SOCKET_API to be defined to be able to
use various POSIX versions of networking functions.  Notably sendmsg()
resulted in "sendmsg() failed (9: Bad file number)" alerts without it.

See xopen_networking(7) for more details.

Poll event method needs ngx_cycle->files to work, and use of ngx_exit_cycle
without files set caused null pointer dereference in resolver's cleanup
on udp socket close.

This includes trailings dots and spaces, NTFS streams (and short names, as
previously checked).  The checks are now also done in ngx_file_info(), thus
allowing to use the "try_files" directive to protect external scripts.

After jemalloc 3.0.0 import there is no _malloc_options symbol, it has
been replaced with the malloc_conf one with a different syntax.

In case of EMFILE/ENFILE returned from accept() we disable accept events,
and (in case of no accept mutex used) arm timer to re-enable them later.
With accept mutex we just drop it, and rely on normal accept mutex handling
to re-enable accept events once it's acquired again.

As we now handle errors in question, logging level was changed to "crit"
(instead of "alert" used for unknown errors).

Note: the code might call ngx_enable_accept_events() multiple times if
there are many listen sockets.  The ngx_enable_accept_events() function was
modified to check if connection is already active (via c->read->active) and
skip it then, thus making multiple calls safe.

Not using full chain passed is ok as consumers are expected to check
event's ready flag to determine if another call is needed, not the
returned size.

We now stop on IOV_MAX iovec entries only if we are going to add new one,
i.e. next buffer can't be coalesced into last iovec.

This also fixes incorrect checks for trailer creation on FreeBSD and
Mac OS X, header.nelts was checked instead of trailer.nelts.

The "complete" flag wasn't cleared on loop iteration start, resulting in
broken behaviour if there were more than IOV_MAX buffers and first
iteration was fully completed (and hence the "complete" flag was set
to 1).