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).

preceding the function.  No functional changes.

POSIX doesn't require it to be defined, and Debian GNU/Hurd doesn't define
it.  Note that if there is no MAX_PATH defined we have to use realpath()
with NULL argument and free() the result.

Most of the systems have it included due to namespace pollution, but
relying on this is a bad idea.  Explicit include is required for at least
Debian GNU/Hurd.