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.

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.

This allows to run nginx with "master_process off" under Windows.

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.

Found by Veracode.